The club confirmed he had joined them via their official Twitter account:
And following the completion of the club record £37.1 million deal, Moyes told United's official club website:
I want to build an exciting team. That’s always the way I’ve wanted my football to be played and I’ve not known it any other way.
I want to win and give the supporters something to shout about because I’ve not done that often enough. What I will say is that this is the first of many more to come in time. I’m looking forward to Juan being the start of it.
United fans will be encouraged by Moyes' claim that he wants to bring in lots more new players to the club. One criticism regularly levelled at the side this season is that there is too much deadwood and not a strong enough depth of talent.
As reported by BBC World Service, via ESPN, former Manchester City manager Roberto Mancini said recently that United need to overhaul their squad in order to achieve more success: "Chelsea changed and bought some new players, City bought four or five new players. After many years, it's the time [for United] to refresh the team."
Moyes has already started that process off by loaning Brazilian midfielder Anderson to Fiorentina, while, as reported by Bryan Swanson of Sky Sports, Fabio is close to joining Cardiff permanently:
Fabio, Wilfried Zaha & Kenwyne Jones all having medicals for Cardiff at the moment. More on @SkySportsNews— Bryan Swanson (@skysports_bryan) January 26, 2014
And while Moyes is keen to bring new players into Old Trafford, it will also do the side a lot of good to move some players on who have not performed sufficiently this season, or in recent years.
The likes of Nani, Ashley Young, Alexander Buttner, Tom Cleverley and Rafael could all be moved out of the club in order to freshen it up.
The likes of players Moyes will want to bring in will be the type who can help United finish in the top four this season, and longer-term challenge for the Premier League title.
The signing of Mata is likely to attract other big-name players to the club as it shows ambition and a desire to win trophies.
Gary Jones in the Daily Star reports that United will move for Southampton's Luke Shaw, an excellent potential signing as a replacement for Patrice Evra. Per Joe Mewis at the Mirror, Newcastle midfielder Yohan Cabaye is another target.
That type of ambition hints that after a turbulent transfer window in the summer and a tough start to the season, Moyes is beginning to find his feet at United and will help the club return to former glories.