Carney Chukwuemeka has undergone surgery on the knee injury he sustained playing for Chelsea against West Ham on Sunday.
The PA news agency understands 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 £20million, 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He had surgery on Monday night after being withdrawn 15 minutes from the end of his side’s loss in east London, the first defeat of Pochettino’s tenure.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.