Mustafi's absence saw the Gunners shift to a four-at-the-back formation, but they could not make a turnaround as they eventually suffered a 3-1 defeat at the hands of United.
Arsene Wenger believes Mustafi will definitely be fit for the midweek London derby with West Ham United next Wednesday [13 December] night if he is unable to feature at Southampton. "We have just a small problem with Mustafi from Saturday, but everybody else looks alright. It's a possibility - if not Southampton, it will be [for] West Ham", he is quoted as saying on Arsenal's official website.
On Thursday, however, they play host to a BATE Borisov side who need to pick up three points to keep alive their hopes of reaching the round of 32.
Wenger also confirmed that Jack Wilshere and Theo Walcott would both start on Thursday [7 December] as Arsenal continue their policy of rotation having already sealed top spot in the group with a match to spare despite last month's shock defeat in Cologne.
"The majority voted for the Champions League teams to go back to the Europa League and [in] many, many years, they won it", he said.
When asked his views on Wilshere's family trip to an open air ice-skating rink this week Wenger replied: "It's not ideal".
With all that in mind, I think that Arsenal will be good enough to win here and get three points on the board.
"They have to make their decisions, I have to make mine", Wenger said of worldwide managers. That's what I was told. "That's part of football".
David de Gea, in particular, was scintillating in the opposition end, and Wenger will want a strong response from whichever team he sets up to face BATE in the final Europa League group game.