Thomas returned to the Arsenal line-up on Sunday for the first time since the start of November, but was forced to leave the pitch just before half-time.
The Ghanaian felt more discomfort in his thigh, which he initially injured against Aston Villa and questions have been raised over whether he was rushed back into action.
The midfielder has undergone scans on his thigh and the club are now trying to determine the extent of his injury.
The Gunners are waiting for the results of the scans and are hoping that the injury is not serious, as they prepare for a busy festive period.
Thomas will not be travelling with the Arsenal squad to Ireland for their Europa League game against Dundalk on Thursday night.
Mikel Arteta is likely to provide an update on the midfielder’s injury later in the day during a pre-match press conference.
Arsenal triggered the release clause in Thomas’ contract to sign him from Atletico Madrid in the last window, and he was expected to make a big impact on the team.
But he has struggled with injuries in an Arsenal side who have been on a dreadful run of form that has seen them drop down to 15th in the league table.
