Chelsea winger Damien Duff will be out of action for at least two weeks after he dislocated his shoulder in the 1-0 victory over west London rivals Fulham in the English Premier League on Saturday.
The Ireland international sustained the injury when he landed heavily after being challenged as he shot at goal in the fifth minute.
"I think he will be out for two or three weeks," Chelsea coach Claudio Ranieri told the club's official website.
"Of course, it is a big blow over Christmas because Damien is Damien. When I don't put him in the squad, my mother who is 84 years old asks why? She kills me about it, it is true!"
Chelsea's win took them joint top of the table on the same points and goal difference as Arsenal.
But Manchester United can go clear at the top if they beat Tottenham Hotspur at White Hart Lane later on Sunday.