United will have to figure out how to use Mata when Rooney returns.
However, just as Mata brings sheer quality to United, he also crowds up an attack that already boasts a plethora of options when healthy.
Naturally an attacking central midfielder, Mata played in that role in his first game for United against Cardiff City. Though he dropped back farther than he normally does in this game, Mata usually sits in the space between the striker and the midfield, feeding teammates and creating chances.
However, Wayne Rooney, formerly an out-and-out striker, has evolved into the same creative role over the past couple of seasons upon the arrival of Robin van Persie. The Dutchman instantly took over the burden of being the team's main goalscorer, while Rooney reinvented himself and amassed 12 assists.
As Rooney and van Persie have become comfortable in these roles, it is believed that Mata will have to be the one to adapt.
More than likely, Mata will spend some time on the right in an inverted winger role. He spent eight games this season there for Chelsea, per WhoScored, and David Moyes has already praised his versatility:
Moyes on Mata: "I can play him in any role. Left or right allows me to keep the Robin & Wayne partnership. I'm quite comfortable with that."— Manchester United (@ManUtd) January 28, 2014
Assuming Manchester United maintain their current 4-2-3-1 formation, Mata will replace Antonio Valencia on the right flank, who has played pretty well this season. When playing as a right attacking midfielder, Valencia has posted a WhoScored rating of 7.33.
While Mata is undoubtedly a great player, it is a big risk to hope that he pans out playing a good chunk of time in a position he is not that used to. He will get to play some games in his preferred central role, but probably not for reasons other than injuries or squad rotation.
However, it is possible that Moyes may later feel the need to switch to a two-striker formation to accommodate Mata. A formation of this sort would allow Rooney to play as a true striker alongside van Persie, and Mata would shift over to the middle to his playmaking role.
If United could land a true ball-winning midfielder in the coming days, such as Inter Milan's Fredy Guarin, then a 4-3-1-2 formation could be a real possibility when the squad is healthy (The Mirror).
For now, Mata will enjoy his time in the middle, until Rooney returns from a groin injury and pushes him out to the wing.
For £37 million, he should be able to make it work.
All stats and info via ESPNFC unless otherwise indicated.