The Chilean has decamped to Inter for the rest of the season after finding himself an odd man out at Old Trafford.
Despite his track record as a consistent goalscorer for Barcelona and Arsenal, Sanchez failed to impress with United following a high-profile arrival 19 months ago.
He was recruited by Jose Mourinho to bring some much-needed pep to the Red Devils forward line, in a deal that saw Henrikh Mkhitaryan go to Arsenal, but struggled to nail down a regular place.
It was a similar story when Mourinho lost his job and Ole Gunnar Solskjaer stepped in, with Sanchez unable to force himself ahead of the likes of Marcus Rashford and Anthony Martial in the manager's plans.
"Sometimes I'd play 60 minutes, then I wouldn't play the next game and I didn't know why."— BBC Sport (@BBCSport) September 3, 2019
Alexis Sanchez's spoken about his time at Manchester United. pic.twitter.com/GYv4P4v2p2
More recently, even teenager Mason Greenwood appeared to have edged ahead of Sanchez, leaving the 30-year-old with little option but to look elsewhere for first-team opportunities.
Sanchez says he remains "very happy" about his decision to join United and he feels he deserved more opportunities to make an impact.
"I think that I'm happy when I play for my national team," he told BBC Sport.
"I was happy at Manchester United too, but I've always said to my friends: I want to play.
"If they would let me play I'll do my best. Sometimes I'd play 60 minutes then I wouldn't play the next game - and I didn't know why."
Sanchez was earning wages that were said to outstrip those of all his United's club-mates, making his decision to join the 20-time English league champions one that was bound to attract scrutiny.
He now joins another former United star at San Siro in Romelu Lukaku, who made a permanent switch in early August.
"I'm very happy I went to Manchester United," Sanchez said. "When I went to Arsenal it was fantastic - I was happy there - but United were growing at the time, they were buying players to win something.
"I wanted to join them and win everything. I don't regret going there."