The Argentina international had expected to move to United in the summer before Sir Alex Ferguson's retirement put a hold on all major transfer activity at Old Trafford.
Garay admitted earlier this season: "I believed I was going to join Manchester United in the summer.
"The deal was nearly closed but finally Benfica did not accept the offer.
"I believe the exit of Ferguson has influenced the deal. Moyes did not believe in me as much but that's not a problem."
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