Mustafi suffered the muscle problem while on worldwide duty with Germany against Azerbaijan as the holders secured their place at the 2018 World Cup on the back of 10 straight qualifying wins in Group C.
"It is always hard to predict the situation because the player I least expected to lose during the break was Mustafi", Wenger said.
Arsenal will be hoping to extend their winning run after defeating Brighton by a 2-0 score-line in their last game before the global break.
"I will have to speak to him", Wenger said. He didn't play in the first game, and we lost him in the second game against Azerbaijan at home.
"I haven't seen everybody because some players are coming back today, some even tomorrow like Alexis Sanchez", Wenger said on Thursday, when questioned about injuries picked up during the global break.
"I will decide what I have to do when Koscielny has his test tomorrow".
"Yes, I have. I think about sorting out the problem, but overall I will completely decide what I do tomorrow because Koscielny will have a test then". At the moment, he's not available.
Wenger, meanwhile, will make a late decision on the make up of his front three for the game against Marco Silva's men. "But he trained on his own and the first signs are positive so we chose to give him a test tomorrow".