Carney Chukwuemeka has undergone surgery on the knee injury he sustained playing for Chelsea against West Ham on Sunday.
The 19-year-old, who scored his first goal for the club in their 3-1 defeat at the London Stadium, will be out for around six weeks.
The former Aston Villa midfielder struggled for game time last season after joining 12 months ago for £20m, but has started both of Mauricio Pochettino's first two Premier League games in charge.
He had surgery on Monday night after being withdrawn 15 minutes from the end of his side’s loss in east London, the first defeat of Pochettino’s tenure.
Earlier, he had equalised Nayef Aguerd’s headed goal when he cut inside and crashed an unstoppable right-footed drive past goalkeeper Alphonse Areola.
It was his first goal in 17 appearances in all competitions since arriving at Stamford Bridge.
The Hammers went on to win it in the second half with goal from Michail Antonio and a penalty from Lucas Paqueta, after Enzo Fernandez had a penalty saved for the visitors.
Meanwhile, Lewis Hall has joined Newcastle on a year-long loan from Chelsea with the option for a permanent deal for the defender.
The clubs have been in negotiations over a deal for the 18-year-old academy graduate since last week, with Eddie Howe's side having come in with an initial offer of £28m.
Hall agreed a new six-year contract to stay at Stamford Bridge earlier in August but Newcastle’s interest has persuaded Chelsea to part with a player who made 11 first-team appearances last season, including nine in the Premier League.
