"I don't think he will be available before the next international break (in November)," Wenger told a news conference on Thursday.
The manager sounded more optimistic about Koscielny, who was not called up by France for their qualifiers due to a strained Achilles, saying that the defender had trained on Thursday and could step in for Mustafi.
"Koscielny has a definite final test tomorrow," Wenger added. "At the moment he is not available. But he trained on his own and the first signs are positive, so we decided to give him a test tomorrow," he added.
Wenger said forward Danny Welbeck (groin) and midfielder Mesut Ozil (knee) had recovered from their injures and were training, while defender Sead Kolasinac had a minor hip problem.
Alexis Sanchez, who saw his 2018 World Cup dreams crushed when Chile lost to Brazil on Tuesday, will join the group on Friday and Wenger said he would assess the striker's fitness.
"I will have to speak to him," Wenger said. "I am more concerned that yesterday afternoon I watched the whole game Brazil against Chile to see how difficult the game was and I must say he got some special treatment in this game.
"It was a very physical game. Mentally, I will have to assess the situation when he comes back tomorrow."
Wenger said contract talks with German international Ozil, who is in the final year of his deal, were progressing well.
"That is my understanding (that he wants to stay). The fact that we didn't find agreement last year doesn't mean he wants to leave. The player looks happy. At the present time there is no deal close for me to announce."
Eighth-placed Watford have just one win against Arsenal in eight Premier League meetings but will look to build on their decent start. Arsenal are fifth on 13 points from seven games.
(Reporting by Shrivathsa Sridhar in Bengaluru; Editing by John O'Brien)
