Sanchez left the Emirates Stadium in January as part of a swap deal that saw Henrikh Mkhitaryan head the opposite way, ending his three-and-a-half-year spell at the club.
Gunners boss Wenger was responsible for bringing the ex-Barcelona ace to English football, but he was allowed to depart midway through the campaign following a failure to reach an agreement on a new contract.
Speaking ahead of Sunday's clash at Old Trafford, Wenger is quoted by Arsenal's official website as saying: "Of course [it will be a little bit strange to face Alexis]. The only thing that has not changed is the colour, that's still red.
"It's strange for me to see him in a different shirt because he had a strong bond with the club and of course he was here for a few years. He did extremely well for us.
"At the time he did not accept our contract offer and the best possible way we could get something from his departure was to make an exchange for a player of the quality of Mkhitaryan. For him, it was a move to another big club. Overall, both parties had the interest to do what happened."
Sanchez scored 80 times in 165 matches during his time at Arsenal and has two in 14 for United.