Arsene Wenger had admitted it will feel "strange" to come up against former player Alexis Sanchez at Old Trafford on Sunday.
Sanchez left the Gunners for the Red Devils in January in a deal which saw Henrikh Mkhitaryan move in the opposite direction.
The Chilean, 29, was running down his contract at the Emirates Stadium and would have been able to walk away for free at the end of the season.
Once it became clear he would not be signing a new deal, Arsenal acted to bring in Mkhitaryan as Sanchez joined Jose Mourinho at Old Trafford.