The Ghanaian injured his hamstring during last week's defeat to Aston Villa and will miss Sunday's game with Leeds.
“He was devastated, we were all devastated because we know the importance of Thomas and we haven't had him since we signed him, we haven't had him in three games consecutively," said Arteta.
“I think he has only played 90 minutes once since he has come back from injury, so it is a big blow for us.
“But he is adapting to the league, he has been very unlucky as well and he is very frustrated because he is a very competitive guy who wants to feel important and be important for the team and at the moment the injuries are restricting a lot of game time for him.
“It is a difficult one, he felt something during the game and we didn't think it was too bad but after the scan looked a little bit worse.
“He won't be available to play against Leeds and we will see how it evolves in the next two days, but it is a muscular issue."