...so Louis van Gaal really needs to get his thinking cap on. Does he abandon his favourite formation to make room for Adnan Januzaj? It's also big for Arsenal's Germans and Big Sam.
After the departures of Diego Costa, Filipe Luis and Thibaut Courtois to Chelsea, plenty are saying Atletico Madrid won't retain their title. Tim Stannard thinks otherwise...
Here are the Premier League's first five clubs. Part two is on its way...
What they need: A bit of spark to inspire a top-four finish. It's difficult to pin-point exactly where the Gunners need extra quality, as the squad has a range of strengths and weaknesses in every department. Winners and Losers asked whether Theo Walcott has finally developed to the standard required to play for Arsenal or if Arsenal have dropped to the standard of Theo Walcott, and if the forward leaves in January it's imperative the Gunners replace him. There was a backlash from some Gooners after Arsene Wenger claimed supporters are only interested in big names and it's likely he'll need to bring in one or two new faces to keep fans on side. A big-money move for a box-to-box midfielder such as Juventus' Arturo Vidal would significantly improve the first team.
What they're likely to get: Given his frugal persuasion, it's highly unlikely Wenger will make a move for a player of Vidal's reputation and the manager will probably try to plod along with the players he has. A £15million bid for David Villa has been mooted, while Wilfried Zaha and Atletico Madrid forward Adrian Lopez (171 appearances in La Liga, 25 goals and 10 assists) remain targets. However, an offer for promising Southampton left-back Luke Shaw may be the height of Arsenal's business.
What's been said? Arsene Wenger: "It's very difficult. Because the level of expectation is very high. Because people want to come here and they want to see Lionel Messi."
What they need: Men with beards and battle scars. Considering Villa have the youngest team in the Premier League this season, they've arguably done well to keep their heads above water so far. But the festive period brought a run of three heavy defeats - in which Paul Lambert's team conceded 15 goals but failed to score once - before the 2-2 draw with Swansea restored some pride on New Year's Day. It's clear that Villa require more experience and a rumoured £8million move for Joleon Lescott would help to provide extra resilience.
What they're likely to get: Daniel Storey pondered Villa's current situation in more depth here, and it's unlikely that Lambert will be given a transfer warchest in January. If a buyer can be found for the injured Darren Bent (QPR perhaps?) then a move for Lescott might be possible, but if not Villa will have to rely on loans and cut-price deals.
What's been said? Paul Lambert: "Can I sell the club and its young players to an experienced player? I don't see that is a problem. You don't need to be Einstein to try to sell this club. It is a huge huge club."
What they need: Chelsea have already addressed their limited striking options with the acquisition of Demba Ba and that transfer suggests a proposed £46million move for Falcao will have to wait until the summer. A new central midfielder would have been a priority before David Luiz's move into the middle of the park, but if Frank Lampard exits in January (which is unlikely) Chelsea will need to find a replacement. With Ashley Cole also expected to depart at the end of the season, a move for a new left-back is possible.
What they're likely to get: Leighton Baines has been touted as a replacement for Cole, while a move for a wide-forward such as David Villa or Theo Walcott is also thought to be under consideration. But there is a good chance Roman Abramovich will keep his purse in his pocket, having been burned in this transfer window before and with Rafa Benitez's position still classed as 'interim'.
What's been said? Rafa Benitez: "Obviously we need bodies but I am really pleased with the squad, they train well, the commitment is there and they are keen to learn. I think we will do our job (in the window) and hopefully it will be a good job."
What they need: Extra bodies to help in the push for fourth. Everton have had a brilliant season so far, but David Moyes' small squad is slowly being stretched by injuries and the manager will need to add two or three recruits if the Toffees are to maintain their current form. John Heitinga is apparently angling for an exit, but Everton can ill afford to allow players to leave this month.
What they're likely to get: After a net loss of £9.1million for the 2011-12 season was announced at the start of January, Everton barely have a pot to p*ss in this month and Moyes has already accepted that loans are the way forward. Out-of-favour PSG striker Guillaume Hoarau is an option, while Landycakes Donovan would swim the Atlantic for another spell at Goodison.
What's been said? David Moyes: "My intention is not to sell anyone this January. Because of that what I'll be hoping to do is pick up a couple of loan players or maybe a Bosman if there is anything out there, although at this time of year you rarely get anything of the right quality."
What they need: Fulham failed to replace Mousa Dembele in August and their lack of a midfield linchpin has seen them slip to 13th in the Premier League following a dismal run of just two wins in 13 matches. Martin Jol's summer signings have failed to impress - he recently expressed his disappointment in Kieran Richardson, while Hugo Rodallega has just two goals in 19 appearances - and the manager must rectify his mistakes if Fulham are to repeat their top-half finish.
What they're likely to get: Mohamed Al-Fayed is rather less of a spendthrift these days, but may open the purse strings at the end of the month if Fulham drop any lower in the table. Jol has previously demonstrated an eye for shrewd signings and he will need to be clever as he looks to inject life into an ageing team. A loan move for Dnipro midfielder Derek Boateng sounds like a gamble, but the improving Jordan Henderson - who turned down Fulham in the summer - could boost the Cottagers if he were to join until the end of the season.
What's been said? Martin Jol: "If we can do something in the market, we will try to that. But it's not easy. You know as well as I do that clubs will wait and wait until you have to pay the maximum price and we will not do that."
Matthew Stanger - follow him on Twitter