Former Chelsea midfielder Gus Poyet says the club must make a signing in the January transfer window to avoid "going backwards", but warned the Blues must also be careful to not upset their promising youngsters.
Chelsea boss Frank Lampard was unable to sign any players in the summer due to a two-window transfer ban.
However, that was reduced on appeal, meaning players can now be signed.
"The transfer window will be very important," said Poyet.
"Everything is all very good at the moment at the club but this is Chelsea. He is the manager of Chelsea and at this club you need to get to certain points and this transfer window could be key to that.
"You need to bring in a very special player but you cannot upset the atmosphere. You cannot take away one of the youngsters because otherwise you contradict yourself.
"At the same time you need something to help you get better because something is still missing there. There are some very important decisions to be made."
Chelsea have been linked with a move for a striker with reports suggesting Oliver Giroud is set to leave.
Speaking on the BBC Radio 5 live Euro Leagues podcast, Poyet added: "If Giroud leaves, probably he will bring someone in.
"I don't know how the staff are looking at Michy Batshuayi - if they are seeing him as a replacement if Tammy Abraham can't play.
"But then the question is who do you bring in? If you bring someone to play you will upset Tammy.
"It is tricky but you have to add. If you stand still you go backwards because the rest keep moving forward so you have to be very careful."
'Eriksen situation needs resolving quickly at Spurs'
Another club likely to be active this month is Chelsea's London rivals Tottenham.
Midfielder Christian Eriksen has been strongly linked with a move away from Spurs for some time, with latest reports suggesting Inter Milan are keen on the Denmark international, whose contract runs out in the summer.
Poyet, who played for Spurs between 2001 and 2004, believes the situation needs to be resolved quickly to enable both the club and the player to move on.
"I have been watching Christian and something is obviously not right," Poyet said. "He is not the Eriksen of two years ago. He is not affecting the game in the way he was affecting it.
"I think they need to sort it out. I didn't see Christian Eriksen happy in the FA Cup game with Middlesbrough and I didn't see the fans happy with him.
"It is something that needs to be sorted soon for the best of everybody."