Injury meant the Chile international could only play in the Gunners' win over Cologne, scoring the second goal as they came from behind to record a 3-1 victory.
UEFA rules would prevent Sanchez from lining out from another team in the Europa League, but luckily for United, that 90 minutes against the Germans in September does not rule him out of featuring in the Champions League.
However, Red Devils boss Jose Mourinho still needs to select Sanchez in his squad for the knockout stage of the tournament. Rule 43.01 states that "For all matches from the start of the round of 32, a club may register a maximum of three new eligible players for the remaining matches in the current competition."
Only one of these new players can be a player that has already participated in the Europa League this season, and no player that has played in the Champions League for another club in 2017-18 can be part of the knockout stage squad with a new team.
Mourinho must submit his updated squad list of 25 players by midnight CET on February 1, the day after the transfer window closes. Sanchez will replace a member of Manchester United's original squad list, with James Wilson a possible choice due to his loan transfer to Sheffield United.
Henrikh Mkhitaryan was also part of the squad for the group stage, and his move to Arsenal means that Mourinho will have another spot to fill for the rest of the Champions League campaign.
Sanchez has a good record in the Champions League, having played for both Barcelona and Arsenal in the competition. He made four continental starts in his debut season after moving to Camp Nou from Udinese, netting two goals.
His best return in the Champions League came in 2014-15 when he scored four times for the Gunners and netted three times in each of the following two seasons.