Should Chelsea be prepared to sell, Manchester United won't get a better option than Juan Mata in this transfer window. He's a fantastic player, and United have been lacking a central playmaker for years now.
United's midfield has not been good this season. There's not enough creativity coming from central of wide positions, with the wingers also failing to deliver. Mata would add to that, and the deal makes perfect sense for both United and Chelsea.
For some reason, Mata does not appear to be in Jose Mourinho's plans. There's the suggestion he's not wanted at Chelsea, and the club have the chance to make as much as £40 million by selling him. That money would help recoup some of the fees they've paid on other players; it could also go towards signing somebody else.
United should do all they can to get Mata—even if it means paying a little more than they should, which is highly likely given their desperation. It's very hard to make big deals in January, and Mata is perhaps the one top-class player David Moyes could get in this window.
Even if the deal came off, however, I don't think signing Mata alone would be enough to get United into the top four. Moyes needs three or four players to have a shot at that.
Looking at the squads competing for fourth spot—behind Manchester City, Chelsea and Arsenal—I think Tottenham could be the strongest on paper. Liverpool's squad isn't six points better than United's, and Everton's isn't five points better. That tells me United are underachieving.
Some players might think twice about signing for United now. There's more money elsewhere, and United can't guarantee Champions League football next season. That was a big problem at Liverpool and something they've struggled with.
When I joined Liverpool, in 1999, they might not have been in the Champions League, but there was a real sense the club was moving in the right direction. Gerard Houllier had a vision, had made six or seven new signings, and I fully believed in the cause.
Will players feel the same about United right now? Possibly not. And that could make it very hard for Moyes to attract the players he wants—be it this January or in the summer window. All the more reason to go bold for Mata while he has the chance.
As for Wayne Rooney, that's a separate issue. I don't think Mata signing from Chelsea will have an impact on Rooney potentially joining Mourinho next season. Each transfer stands alone.