The Arsenal midfielder was withdrawn in the second half of the 3-0 defeat on Monday, with manager Mikel Arteta saying he had a problem in the same area as a previous injury.
"Thomas has since received further assessments, and a subsequent scan has shown muscle damage to his right thigh," Arsenal said in a statement.
"We will continue to assess Thomas in the coming weeks, during which time he will also receive further specialist consultations."
The injury is a major blow to the north London club who also lost left-back Kieran Tierney for the remainder of the season with the Scotland international requiring surgery on his left knee.
Arsenal are fifth in the standings with 54 points and a game in hand, behind fourth-placed Tottenham Hotspur on goal difference. Arteta's side next host Brighton & Hove Albion on Saturday.