"I think we said when we come in that we'd have a difficult summer, that period in the transfer window was going to be tough, but we've had a chance to look at everybody and we're only in the next window now and I've already said we'd be making changes - and Juan is the first," Moyes told the press at Mata's unveiling.
He added: "There'll be more players like Juan in the future, I've no doubt about that."
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The Scot also admitted he never expected to be able to complete the signing of the former Chelsea playmaker.
"I'm thrilled," he said. "I got wind a month or two ago that it could be a possibility and I never really believed we'd pull it off, and as I've been saying I didn't think we'd get a chance to do much business in January.
"I didn't think the players would be interested or be available. But when we got a sniff Juan could be available we went after it right away. And congratulations to the club for pulling it off."
While Mata himself described his last six months at Chelsea as difficult and expressed his delight after been given the chance to restart his career at Old Trafford.
"Coming here is a great moment for me. I feel very happy to be here, to have the chance to play in a massive club like this," he told reporters.
"Obviously the last six months have been difficult at Chelsea but now I'm happy to come here and to win as many trophies as I can. I'm sure this will be a great, great period for me.
"The history of the club, the players are top players in the squad, the welcome was unbelievable - I came here by helicopter and I have to say everyone was amazing, the players, the manager, the fans has been unbelievable.
"I'm really grateful and the only thing I want to do is just play, to try my best for the team, and to get this confidence back."