The highly rated Argentina international has long been linked with a move to the Premier League champions but things changed when David Moyes replaced Sir Alex Ferguson.
"I believed I was going to join Manchester United in the summer," Garay said.
"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.
"I am happy at Benfica and have a lot of dreams and ambitions for this season. Now I only think about the present which is Benfica but it's never possible to discard anything in the future."