No winter break, a shorter season and injuries piling up.
Whether a team is challenging for the title, aiming for a European spot or looking to avoid relegation, reinforcements may well be needed for that push in the second half of the season.
Will clubs decide to splash out or aim to bring in players on short-term deals? BBC Sport takes a look at players who could move with the transfer window now open.
PAUL POGBA (27, midfielder, Manchester United)
Linked with: Juventus
Will he leave? Won't he leave? It has been a running theme during Pogba's time at Manchester United and this transfer window is no different.
The France World Cup winner has been a regular performer for United this season, making 19 appearances, but his agent said earlier this month the player is "unhappy" at the club and "has to change teams" in the next transfer window.
MESUT OZIL (32, midfielder, Arsenal)
Linked with: Fenerbahce, DC United, LA Galaxy, Inter Miami
Ozil has been banished by manager Mikel Arteta, failing to make Arsenal's squad for both the domestic and European campaign this season.
A World Cup winner with Germany in 2014, Ozil is one of the Gunners' highest earners. His contract runs out next summer but he may leave earlier if a deal can be agreed.
DELE ALLI (24, midfielder, Tottenham)
Linked with: Paris St-Germain, Everton
England international Alli's future at Tottenham has become increasingly uncertain; he has fallen out of favour under Jose Mourinho and made just one Premier League start so far this season.
Paris St-Germain tried to sign Alli last summer but owner Daniel Levy refused to agree a deal. The French champions look set to make another attempt in January.
OLIVIER GIROUD (34, striker, Chelsea)
Linked with: West Ham, Atletico Madrid, Marseille
Olivier Giroud's Chelsea career seemed to be nearing an end this summer as boss Frank Lampard bolstered his attacking options with the likes of Timo Werner.
However, the veteran France striker has been in great form for club and country, scoring 14 goals in 23 appearances - including all four in a 4-0 win against Sevilla.
That form has apparently alerted a number of clubs including Atletico Madrid. The Spanish side released Diego Costa recently and could see Giroud as the ideal replacement.
DIVOCK ORIGI (25, striker, Liverpool)
Linked with: Wolves, Newcastle, Leeds
Anfield cult hero Origi has seen his game time reduced at Liverpool following the summer arrival of Diogo Jota, playing just eight games and scoring one goal.
Wolves retain a long-held interest and may well bid as they look for cover for striker Raul Jimenez, who is recovering from a fractured skull.
FIKAYO TOMORI (22, defender, Chelsea)
Linked with: Leeds
England international Tomori made 22 appearances for Chelsea last season but has barely featured this term, playing just three games in all competitions.
Leeds looks the likeliest destination with the West Yorkshire side needing cover for long-term absentee Robin Koch.
ADAMA TRAORE (24, winger, Wolverhampton Wanderers)
Linked with: Leeds
Adama Traore had a hugely impressive season in 2019-20 but has not quite been able to carry that form into this campaign and has struggled for game time this season.
The Spain international has started just nine games this season and has failed to score a goal or provide any assists. With Euro 2020 on the horizon, a move to pastures new might be what the winger needs to rediscover his form.
OLEKSANDR ZINCHENKO (24, full-back, Manchester City)
Linked with: Southampton, Wolves, Leicester, West Ham
Ukraine full-back Zinchenko has fallen down the pecking order at Etihad Stadium, playing just 12 Premier League minutes this season.
With no guarantee of first-team football, City "will listen to offers" for the player and will not be short of suitors.
YVES BISSOUMA (24, midfielder, Brighton)
Linked with: Liverpool, Manchester United, Monaco
Mali midfielder Bissouma has been a regular in the Brighton line-up since signing from Lille in 2018.
But with Georginio Wijnaldum yet to sign a new contract at Liverpool, Bissouma has been touted as a possible replacement for the Dutchman.
JAMES TARKOWSKI (28, defender, Burnley)
Linked with: West Ham, Leicester
Centre-back Tarkowski has been a firm fixture in the Clarets backline and has also gone on to win two England caps.
But he has so far failed to agree a new contract at Turf Moor, with his current deal expiring in 18 months' time.
JAMES MCCARTHY (30, midfielder, Crystal Palace)
Linked with: Celtic, Aston Villa, Burnley
Republic of Ireland international McCarthy has failed to hold down a first-team place at Crystal Palace and has played just four games this season.
The experienced midfielder has played more than 250 top-flight games but is out of contract at the end of the season and could secure a move away in January.
JOSHUA KING (28, striker, Bournemouth)
Linked with: Aston Villa
King almost moved to Manchester United last summer but ended up staying with Bournemouth following relegation to the Championship.
The Cherries are flying high in the second tier and could cash in on King, whose contract runs out at the end of the season.
EMILIANO BUENDIA (24, midfielder, Norwich)
Linked with: Arsenal, Atletico Madrid, Valencia
Buendia has been in superb form this season, scoring six goals and providing six assists as Norwich look to bounce back to the Premier League at the first attempt.
The Argentine may well be looking for a return to the top flight and would be a creative force for Arsenal, who are in desperate need of inspiration in the final third.
MATTHEW SORINOLA (19, defender, MK Dons)
Linked with: Leeds, West Ham
Teenage defender Sorinola has caught the eye in his breakthrough season for MK Dons in League One, playing 19 games.
A host of scouts have been impressed with his progress and a number of clubs are said to be chasing his signature next month.
ODSONNE EDOUARD (22, striker, Celtic)
Linked with: AC Milan
Many expected the France Under-21 international to leave in the summer - and Edouard himself was among them. As transfer noise buffeted around him, the striker's early-season form was underwhelming at best. No offers were forthcoming before the summer window closed and Edouard has been some way off his best ever since.
Celtic are unlikely to be searching for suitors but, should one come along with an enticing offer, it is entirely possible a deal could be done.
ALFREDO MORELOS (24, striker, Rangers)
Linked with: Lille
This time last year, the Colombian was in fine fettle. Goals followed goals in the opening half of the domestic season and the striker was rampant in the Europa League, too. Clubs across the continent cast covetous glances. Then it stopped. The goals dried up as Rangers' title challenge faltered.
Lille were sniffing around in the summer and, with Rangers eager to generate revenue from player sales, Morelos would seem like a likely candidate to depart after a frustrating opening half of this season.
DAVID ALABA (28, defender, Bayern Munich)
Linked with: Manchester United, Manchester City, Chelsea, Barcelona, Real Madrid, PSG
Austria international Alaba won the Treble with Bayern Munich last season but is in the final year of his contract and has yet to agree a new one.
The defender can start talking to other clubs in January. A host of top clubs across Europe are interested in his signature.
PAPU GOMEZ (32, forward, Atalanta)
Linked with: Juventus, Inter, AC Milan, Napoli, Roma, PSG
The Argentine attacking midfielder appears to have fallen out with his coach Gian Piero Gasperini. He's 32 years old but is one of Atalanta's most important players and a huge part of their recent success in the Champions League, reaching the knockout stages in the last two seasons.
CHRISTIAN ERIKSEN (28, midfielder, Inter Milan)
Linked with: PSG, Arsenal, Manchester United
The Denmark star's move to Italy from Tottenham 12 months ago has not worked out and he has a strained relationship with Inter head coach Antonio Conte. At 28, he is at a key moment in his career and wants regular first-team football.
HOUSSEM AOUAR (22, midfielder, Lyon)
Linked with: Real Madrid, Juventus, PSG, Arsenal
France midfielder Aouar was strongly linked with Arsenal in the last transfer window.
With Lyon challenging PSG for the title this season, he is more likely to move in the summer than in January.
LEANDRO PAREDES (26, midfielder, PSG)
Linked with: Inter Milan
According to the Italian media, a swap deal involving Marcelo Brozovic and Leandro Paredes collapsed on deadline day in October. Paredes was in and out of the PSG starting XI before manager Thomas Tuchel's recent sacking, and they could let him leave in order to free up space and money to bring in one of their targets.
ISCO (28, midfielder, Real Madrid)
Linked with: Sevilla, Juventus, Arsenal, Everton
Zinedine Zidane no longer seems to consider Isco as one of his key players and Spanish media are reporting that the 28-year-old sees his future outside La Liga. His contract doesn't expire until 2022 so anyone interested would still need to pay a considerable fee.
DIEGO COSTA (32, striker, free agent)
Linked with: Arsenal, Wolves
Costa's departure from Atletico Madrid in December has paved the way for a possible Premier League return.
However, the 32-year-old will be keen for regular football after making just three substitute appearances since mid-October. The Spain striker scored 59 goals in 120 games for Chelsea between 2014 and 2017 and still has the aggression to upset defenders even if he is not as prolific as he once was.
MERGIM BERISHA (22, striker, RB Salzburg)
Linked with: Parma, Sassuolo, Torino
When it comes to this time of year, RB Salzburg are European football's top supermarket. Takumi Minamino and Erling Braut Haaland left the Austrian club 12 months ago and Dominik Szoboszlai has already agreed his move to RB Leipzig, to go through officially once the January window opens.
Berisha has caught the eye, notably with goals against Bayern Munich and Lokomotiv Moscow in the Champions League.
RENATO SANCHES (23, midfielder, Lille)
Linked with: Liverpool, Inter Milan
The Portugal international midfielder, who burst on to the scene as a teenager at Benfica, has rebuilt his reputation after life at Bayern Munich didn't work out.
Like many French clubs, Lille are having financial difficulties and with new owners in charge he may be allowed to leave to raise funds.
ARKADIUSZ MILIK (26, striker, Napoli)
Linked with: Roma, Juventus, Fiorentina, Everton, Tottenham
The Polish striker was omitted from Napoli's Serie A and Europa League squads this season after a contract dispute. He has scored 48 goals in 122 appearances across all competitions for the Italian club, having joined from Ajax in 2016.
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