United, who won the title last season, are languishing in the seventh position this campaign as David Moyes has struggled to replace Sir Alex Ferguson.
Mata joined United from Chelsea for a club record fee of £37.1 million in January but said that he knew what he was getting into when he joined and that the promise of future signings convinced him to move up north.
"When I joined, in all my conversations with the board and the manager about next season, they were confident about getting the right players," he told the Mirror.
“They know, and I know, we’re in a transitional moment, but in the summer they’re going to try to sign a few players and I have the faith they are going to bring top players in.
“I’m sure this club is going to come back and fight for every trophy. Since I was a kid, Manchester United was fighting for everything, and this moment is going to come back for sure.
Mata also said that he was fully behind Moyes and that people blaming the Scot for United's poor season are being unfair.
"We have to be together. Now is not the time to blame anyone. Now is not the time to look for guilty people," he said.
“Now is the time to stick together - with the manager, with his staff, the players and with the people on the board of the club. We need to carry on, to stay with the manager. We need to believe in him, because as players we know we have to improve.
“It’s not just the manager, it’s not just the tactics. It’s all of us. It’s not the time to blame one person. We are all together in this.
"It’s not a team that’s just played one season in the Champions League and you’re going to a club that’s not top level. No, this will always be a top level club, one of the biggest in the world.
"So if you don’t play Champions League one season, you’ll play in it the next year, or you will still win trophies, because it’s what the club is about."
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