Christian Pulisic missed Chelsea's routine 2-0 win against Nottingham Forest in the FA Cup third round because of an unspecified injury sustained in training on Saturday.
Frank Lampard confirmed the issue to the club's in-house TV channel but did not specify what the injury was or the severity.
The American had started the New Year's Day draw against Brighton and Hove Albion having missed the win away to Arsenal because of a minor hamstring complaint that Lampard said was more like a cramp than a proper injury.
In Pulisic's absence Chelsea won comfortably at home for the first time since the autumn as Callum Hudson-Odoi and Ross Barkley found the net in the first half against a Forest side that had been much weakened with their priority being a promotion push in the Championship.
Lampard's side now have six days off before returning to Premier League action, when they welcome Sean Dyche's Burnley to Stamford Bridge next Saturday.
By that point there may be an update on the future of Olivier Giroud, who was an unused sub today and is keen to leave the club in search of regular game time.
The head coach said he can leave if the club can sign a new attacker in the January transfer window. Chelsea have been linked to a host of names including Moussa Dembele, of Lyon, and Timo Werner, of RB Leipzig.
The FA Cup fourth-round draw takes place on Monday night during TV coverage of Arsenal's game at home to Leeds United.
