Chelsea boss Maurizio Sarri says it will be "not be easy" to challenge Liverpool and Manchester City next season, after an unsuccessful appeal against Fifa's transfer ban.
The Blues are banned from signing players during the next two transfer windows until the end of January 2020.
It followed an investigation into their signing of foreign under-18 players.
Chelsea said they were "very disappointed" and will now appeal to the Court of Arbitration for Sport.
"It's very difficult to cover the gap at the moment; we need to work, probably we need to do something from the market," said Sarri.
"So it's not easy, because the level of the top two is very, very high."
Sarri says the club need to buy two players this summer.
"I think that we have to buy only one, two players, otherwise it's very difficult to improve immediately," the Italian added.
"I think we are a very good team, so we need only one, two players, no more."
The only change to the original Fifa ban is that the Blues can sign under-16 players from the UK during the suspension period.
"The Fifa appeal committee has decided to partially uphold the appeal lodged by Chelsea," Fifa said in a statement.
"This ban applied to the club as a whole - with the exception of the women's and futsal teams - and did not prevent the release of players."
Fifa said it found breaches in 29 cases out of the 92 investigated.
Chelsea's fine of £460,000 by world football's governing body also remains.
The Football Association (FA) was also fined £390,000 by Fifa when it issued the ban and English football's governing body was told that it must "address the situation" regarding the international transfer and registration of minors.
"Chelsea categorically refutes the findings of the Fifa Appeal Committee," the club said in a statement.
'Loan players must be considered'
Chelsea have a number of high-profile players out on loan that they can call on, including Tammy Abraham (Aston Villa), Michy Batshuayi (Crystal Palace), Alvaro Morata (Atletico Madrid), Victor Moses (Fenerbahce), Christian Pulisic (Borussia Dortmund), Kenedy (Newcastle), Tiemoue Bakayoko (AC Milan) and Kurt Zouma (Everton).
They also have dozens of youth players out on loan.
"Every month I have a report on every player on loan; we have about 45," Sarri said.
"There are two or three players who have been out on loan this season who are interesting. I do not want to name names now. But the level they have been playing at must be considered."
How did this come about?
Based on documents from Football Leaks, French website Mediapart claimed in November that 19 Chelsea signings had been looked at during a three-year investigation.
Mediapart alleged that 14 of those signings were under the age of 18.
It was first reported in September 2017 that Chelsea were being investigated.
Burkina Faso international Bertrand Traore - who now plays for Ligue 1 club Lyon - signed his first professional contract at Chelsea in 2013 at the age of 18 but was not registered until January 2014.
Mediapart claimed Fifa found evidence that Chelsea had misled them over the dates, while Traore was found to have made 25 appearances for the Blues (under-16, under-18 and first team) despite not being registered by the FA.
Chelsea admitted they paid his mother £155,000, as well as a further £13,000 to the club she chaired - AJE Bobo-Dioulasso - in April 2011 to allow them first refusal over his signature.
That deal, it is alleged, was for four-and-a-half years, despite the limit for under-18s being three years.
In addition, it is also claimed Chelsea paid for Traore to attend the £20,000-a-year Whitgift School in Surrey.
Real Madrid and Atletico Madrid both received bans for breaching rules over the signing of minors in early 2016, while fellow Spanish club Barcelona were given a 14-month ban after breaking rules for signing international under-18s in 2014.
However, a Barcelona appeal saw their punishment pushed back a year, allowing the club to sign Luis Suarez, Ivan Rakitic, Jeremy Mathieu, Claudio Bravo and Marc-Andre ter Stegen.
What are Fifa's rules?
Fifa bans the transfer of under-18s to different countries unless they meet strict criteria. It brought in the rules to help protect children from exploitation and trafficking.
Under-18s can only be transferred abroad if:
- The player's parents move to the country in which the new club is located for non-footballing reasons.
- Both clubs are in the European Union or European Economic Area and the player is aged between 16 and 18. Even then, the buying club must meet more criteria relating to education, training, living conditions and support.
- They live within 100km of the club.