Arsenal midfielder Lucas Torreira suffered a fractured ankle during Monday's FA Cup fifth round win against Portsmouth.
Torreira, 24, had to be carried off on a stretcher after just 16 minutes following a challenge by Portsmouth defender James Bolton.
No timescale has yet been put on his recovery, with Arsenal waiting for "further specialist reviews".
Torreira has made 33 appearances for Arsenal this season.
"I spoke with him and he was in pain," said Arsenal boss Mikel Arteta.
"We don't really know exactly the extent of the injury - he needs to see the specialist here in London and we will know more about it.
"Since I joined, the amount of things that happened is enormous, but we just have to try to adapt.
"It doesn't only happen to us, unfortunately it also happens to many other teams, so we cannot be crying about that."
Meanwhile, defender Kieran Tierney is back in full training after recovering from a dislocated shoulder.
Injuries have restricted the Scotland international to just 11 appearances for the Gunners since signing from Celtic last summer.
Arsenal are also hopeful Cedric Soares will return to full training next week after recovering from a knee injury, with Sead Kolasinac due to resume training by the end of the month after a shoulder problem.