The Red Devils managed to see off competition from defending Premier League champions Manchester City to sign Sanchez in the January transfer window.
Sanchez scored 80 times in 166 games during three and a half seasons at the north London side following a move to Arsenal from FC Barcelona in a £35m deal in 2014.
Manchester United sent Henrikh Mkhitaryan to Arsenal in a swap deal to secure Sanchez’s signature ahead of the second half of the 2017-18 season.
Sanchez, who is the best paid player at Old Trafford, scored an underwhelming three goals in 18 games in all competitions.
The South American has continued to struggle at the start of the current campaign after failing to find the net in their opening five Premier League fixtures.
Former Arsenal star Merson delivered a withering verdict on Manchester United’s signing of Sanchez.
“You are keeping out a young talent in Marcus Rashford,” Merson told Sky Sports. “But Rashford did play last week in Europe, and he didn’t really shine. Mourinho then has a case of saying ‘you didn’t really turn it on in that game’.
“It is a problem giving him big wages. I don’t know if Man Utd wanted him. I think it was a case of there’s nobody else around, so we’ll take him so Man City can’t have him and it’s backfired in a way.”
Sanchez won two FA Cup titles during his spell at Arsenal but the Chile star struggled to fire the Gunners to a Premier League title challenge.
Manchester United are in seventh spot in the Premier League table and eight points adrift of leaders Liverpool FC.
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