Arsenal midfielder Aaron Ramsey suffered what manager Arsene Wenger described as a "worrying" injury in their 2-1 victory over Leicester City.
Wales international Ramsey, 24, was introduced on 73 minutes but limped off with a muscular problem.
"It's very early for Aaron but he stopped straight away and that is never a good sign," said Wenger.
Alexis Sanchez was also withdrawn but Wenger expects his top scorer to face Middlesbrough in the FA Cup on Sunday.
The former Barcelona player - returning from a hamstring injury - was feeling the effects of a kick on the knee when he left the field for Olivier Giroud on 68 minutes.
Ramsey looked frustrated when he sat on the turf awaiting treatment late on, having replaced Theo Walcott, the scorer of Arsenal's winner.
"He's not overplayed - since he's had these problems I've been more cautious," said Wenger of the midfielder.
"It looks like it [a recurrence of a hamstring injury]. I don't know how bad it is yet. Just visually, it didn't look good."
Ramsey suffered a hamstring injury against Galatasaray in early December which kept him out for six weeks but started four games since returning on 18 January before the visit of Leicester.
The latest set-back will be a worry for Wales manager Chris Coleman, whose team face a Euro 2016 qualifier against Group B leaders Israel on 28 March.