Mustafi was forced off in the first half of Germany's 5-1 World Cup qualifying victory over Azerbaijan on Sunday and is not expected to return until after the next worldwide break.
At his pre-match press conference ahead of the trip to Watford, manager Arsene Wenger has now revealed that the centre-back could be missing until the end of November.
Koscielny, on the other hand, skipped the recent global break to recover from an Achilles injury that has been a long-standing issue for the France defender.
The former Schalke 04 man has made six appearances in all competitions during the current campaign.
"Koscielny will have a test tomorrow", Wenger added.
Meanwhile, Laurent Koscielny and Danny Welbeck are also at risk of missing the game due to injuries.
"I will have to speak to him", Wenger said. At the moment, he's not available. Sanchez won't be back with us until tomorrow.
Alexis Sanchez will be assessed when he returns from global duty after he received "special treatment" following Chile's 3-0 defeat to Brazil.
Chelsea and Arsenal both have very winnable away games in the Premier League, while Fulham will fancy their chances against PNE, so 7/2 (4.50) with dabblebet on the enhanced treble, boosted from 3/1 (4.0), could well prove to be a gift for punters. The fact that we didn't find agreement past year doesn't mean he wants to leave. "At the present time there is no deal close for me to announce".
Mustafi has been central to Arsenal's resurgence of late, starting in each of their last four Premier League games which has seen them keep four clean sheets and pick up 10 points from a possible 12.
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