A lack of first-team opportunities, and concerns over his international future, prompted the attacking midfielder into completing his £37.1 million (44.7m euros) transfer between the Premier League rivals.
Mata, 25, was relaxed and enthusiastic as he was unveiled to the world’s media at United’s training ground on Monday and admitted that the helicopter journey his new employers provided had been one of the highlights of his move.
But it was the more serious prospect of ensuring he added to the 32 caps he has already earned with his national team, that led to the transfer, especially as Mata had been left out of one Spain squad already this season.
“It was really important,” said Mata of the part concerns over an international place played in his move.
“The Spain squad is really difficult to get into, there are a lot of players fighting for positions to be in the last 23 going to Brazil.
“I hope this is a great chance for me to keep playing, keep fit and show the manager of the national team I want to be there, I have to be there.
“I don’t know what would have happened if I had stayed at Chelsea. Now I am here, now the moment is here. Now I’m trying to play as much as I can to help the team and if things are good in the final part of the season, the World Cup is going to be closer for me, for sure.”
Mata’s move from London to Manchester has been one of the more unusual in Premier League history with Blues manager Jose Mourinho content to sell such a highly-regarded player to one of Chelsea’s traditional rivals.
The player himself admitted he was struggling to explain either his move, or the fact that his first team opportunities had all but disappeared since the return of the Portuguese manager to Stamford Bridge.
“It is football. In football everything is possible,” said Mata. “The first two seasons in Chelsea for me were amazing, I settled down so quickly, the fans were unbelievable, the club, everything.
“The last six months were difficult because I didn’t play as much as I wanted but I respect that. Jose is a very good manager, the squad is unbelievable, they have top, top players, I think they have one of the best squads in the Premier League.
“I respected all my team-mates, I just tried my best in training and every game and the minutes I had but when Man United came it was a great option for me to carry on with my career.”
And Mata said he was particularly looking forward to playing in the same side as England striker Wayne Rooney, who is in talks with United over a contract extension.
“I can’t wait to play alongside him,” said Mata. “For me, he is one of the best players in the history of this country, he is unbelievable, he is a striker who can score, can assist and can come to the midfield to take the ball and move so I think I will enjoy playing alongside him.
“I will try to connect with him as best as I can to try and find the gaps between the rival defences because I was doing this in the last season, that is my best kind of play, to be in the position I have to be in to assist the strikers.”