The current Premier League champions were criticised for their lack of spending in the previous window, but have shown their intent this month by securing Mata's signature for £37.1m.
"I wanted to go to a place where I would feel happy and I'm happy now," Mata said in a press conference.
"Manchester United are a massive club, with the most fans around the world so it's a big challenge for my career, a big step.
"With the players that are here and to come in the summer, this club is going to be successful for sure."
United are currently seventh in the league table.