The most important by far, United's long-running weakness, and a position they will probably need two players in. Fortunately, it's also been the one with the most links.
With Wayne Rooney potentially on the way out, some sources have surmised that United will want a big-name signing to replace him, and while Fabregas would be on high wages, the transfer fee would be affordable if Barcelona decide to let him go. A perfect fit for United who can bring playmaking ability, goals, and drive to midfield, a deal will be difficult to strike for obvious reasons, but it seems that there is a serious belief he could be tempted to Old Trafford.
At 18m due to his release clause, it's now or never to sign one of the best young midfield talents in the world, and Thiago will offer a more exciting but also more lightweight option in midfield. With plenty of skills and trickery, the Italian-born midfielder can give United an entirely new dimension, and although he's not an ideal fit, he would be a steal at the price. This appears to be likely.
A hulking bruiser of a midfielder who has a remarkably complete game arguably suits United's needs more than anyone, but there are question marks over whether he has the quality to play at this level. The concerns over his best position are unimportant - he has the attributes to play deeper and was excellent when he did so for Everton. A deal would be relatively straightforward to tie up if David Moyes is prepared to raid his old club, but the Belgian may be sidelined if the other targets are preferred.
A midfielder who has been linked to United before, and would offer the advantage of being able to replace as well as complement Michael Carrick due to his all-round ability. Could be a touch raw and untested at such a high level, but would be a less expensive option than the likes of Fabregas. If United spend heavily elsewhere or miss out on their big targets, this is a likely move.
Rumour has it that Modric, considered a flop by many in Spain, will be shipped out if Madrid can find a buyer, and he would be perfect for United - adept at playing in the middle or further forward, who offers a degree of solidity with a great range of passing and a superb touch. Doesn't have goals to his game as Fabregas does, and is reported to be second-choice for United, but that also depends on Madrid selling. An exciting thought, but there's not much concrete backing it up at present.
The former Chelsea midfielder, who moved to Benfica as a makeweight in the David Luiz deal, has been linked to United but frankly, it's a stretch to consider him the answer to the midfield problems at Old Trafford. Would be a surprise, and a very disappointing acquisition.
With Antonio Valencia out of form, Ryan Giggs approaching 40, Ashley Young mediocre and Nani probably on the way out, United are desperately lacking out wide. Danny Welbeck and Shinji Kagawa will likely find themselves shunted out there enough, and Wilfried Zaha has been added, but United could still do with a genuine option out there. It's a surprise, then, that very few links have materialised.
Although more of a midfielder, Ljajic has played out on the wing a lot at Fiorentina and could be an option there for United. Although the club pulled out of signing him previously, he may jump at the chance to go back for real, and wouldn't be too expensive. The suggestion he'll return isn't well-sourced though, so it may not be very likely.
The big one, with Real Madrid seemingly now insisting their man will stay for at least another year. The talk of an Old Trafford return has cooled lately, although some newspapers have suggested there may have been more than just a back injury to his absence at José Mourinho's final game. Would obviously be a huge transfer and the biggest of the window, and all United's other plans would have to accomodate it, but if Rooney leaves, United can certainly make it happen unless Madrid simply refuse to sell for a year.
Marco van Ginkel
Was a rumoured target of David Moyes at Everton, and although the young Dutchman has plenty of potential, it's hard to see him being good enough to tie down a starting place at United right now. Chelsea have also been linked, but it's hard to see United splashing out on something they won't get much of an immediate return on.
Patrice Evra is rumoured to be joining the mass recruitment at Monaco, and if so then United will definitely need a new left-back. One name has been linked far more than any other.
How much of this is concrete and how much is an assumption Moyes will look to his old club for players is unknown, but Baines was linked to United plenty of times in the past and now could be the time he makes his move. A significant outlay will be needed for a player who isn't exactly a spring chicken, which doesn't fit Glazernomics, but few other options have been suggested.
Known to be scouted by United many times, Alex Sandro would be a more complete, classier option than Baines as well as a younger player for the future. The main problem is that Porto are unlikely to be willing to sell, having already lost two players this year and with other vultures circling and snapping at the likes of Fernando, Eliaquim Mangala and Jackson Martinez. The club will want to avoid a mass exodus, and when it comes to selling players, they usually get their own way. United would probably need to pay a huge fee here, and it's hard to see it happening this year at least.
Rumoured to be on the way out of Madrid, and while United haven't been linked just yet, their past interest and Portuguese connections means it's always a possibility. Would be a good option - wages would be affordable, likely comparable to Patrice Evra's, as would the transfer fee. It would also represent a solid option where United know exactly what they're getting, so if a link does emerge, it makes perfect sense.
Probably a case of two plus two equals five, but Jagielka has been linked to United this window. The logic behind it makes little sense - there's hardly a dearth of experience at centre-back at United, and the Red Devils are probably better-stocked in that position than anyone. Let alone the fact that Jagielka simply isn't good enough to be playing at the highest level, this seems a non-starter.
Apparently green-lighted by David Moyes, a young and somewhat unknown quantity to provide cover at full-back, this deal would be an inexpensive gamble that has little bearing on United's other activity.
Touted as Rooney's replacement, but it's doubtful that United would even need another striker even if Rooney were to leave, let alone spend heavily on an area of the team that is probably their position of least concern outside centre-back and goalkeeper. The rumour appears to have gone quiet with Bayern Munich now in pole position.
Nani has been linked to several clubs, among them Galatasaray, Arsenal, and latterly, Monaco, and the consensus appears to be that the winger is on his way out. Not having managed to tie down a first-team place in the years he's been at the club, his departure shouldn't leave too much of a gap in United's first-team options and will provide extra funds to spend.
Talk of his departure had cooled until rumours surfaced of a massive contract offer from Paris Saint-Germain, and there has been plenty to suggest that United have finally run out of patience with the striker over his attitude and fitness problems. There is a lot of uncertainty over his apparent transfer request, but whether he leaves or not - and it is still not certain at this stage - would dictate much of United's other business.
It's difficult to see Anderson remaining at Old Trafford for too much longer - they must have given up by now - and yet nobody seems prepared to come in for a bid with the injury-prone midfielder. One to watch for later in the window.
Rumour has it that Evra will soon depart for Monaco, leaving United in need of a new left-back, but although his attacking form has recovered, he has never looked like the defender he once was. It'll be sad for United fans to see him go, but it would also be pragmatic.
Lots of big clubs in need of a striker, Real Madrid and Juventus among them, have been linked with a cheeky bid for United's young poacher. If Rooney leaves, he will surely be assured of more game time, and did more than enough last season to confirm that he deserves it. Would be a surprise, and a great shame, if United allowed him to leave.