The Chile international opted to quit The Emirates in the mid-season window after the relentless speculation about his future at the north London side.
Sanchez ended up moving to Manchester United as part of the same deal for Henrikh Mkhitaryan to head to Arsenal as the Gunners managed to convince the 29-year-old to move to London.
The South American attacker will be hoping to help Manchester United enjoy a successful end to the season under Mourinho this term.
And the Portuguese coach has now highlighted Champions League football as one of the main reasons why Sanchez was tempted by a move up north.
Speaking at a news conference on Tuesday night, Mourinho said: “It is true that we have a new player, Alexis Sanchez, and we could have waited for the transfer market in the summer but we decided to go now in the transfer market.
“We felt we needed him. Manchester United in the Champions League is quite a big thing, Alexis came with all his efforts to play on the Champions League stage.
“We know it has been hard for me as a coach to motivate my players for a competition we didn’t want to play [the Europa League].
“But because we won it we got to qualify for the Champions League so motivation is always there – we always want to play against the best teams.
“So Alexis wanted to come to a team to play in the Champions League.”
Manchester United are now preparing for their crunch Premier League clash against Chelsea at Old Trafford on Sunday afternoon.
The Red Devils are currently in second place in the Premier League table as they chase Champions League qualification in Mourinho’s second season in charge at the Old Trafford club.
Manchester United won both the League Cup and Europa League last season.
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