Manchester United took a step back in their progress under Louis van Gaal, when Southampton won 1-0 at Old Trafford on Sunday. Having previously gone 10 games unbeaten in the Premier League, United have now won just once in their last five to drop down to fourth place. Such a decline in form, which leaves United clinging onto the vital final Champions League place, may be enough to convince Van Gaal to act in the January transfer window.

Since the start of his reign, it has been the defense that has looked most in need of strengthening. A deficiency at the back remains, but in recent weeks it has been going forward that there have been greater problems. Radamel Falcao was left out of the squad entirely on Sunday, raising even stronger doubts about whether United will pay the £43 million to sign the Colombian striker on a permanent deal from Monaco. Meanwhile, Robin van Persie continues to look short of his best and Wayne Rooney is now settling into a midfield role.

With United now showing a willingness to spend big in order to sign ready-made stars, could the club be considering a blockbuster deal in the future? That’s the suggestion put forward by The Guardian on Monday, with Barcelona’s Lionel Messi named as a sensational target. Messi’s future has been the subject of much conjecture in the past week after a rift with manager Luis Enrique reportedly led to him missing a training session at Barcelona. And the four time world player of the year caused further scrutiny over his future ahead of the Ballon d’Or ceremony in Zurich with some non-committal comments about his plans for next season.

“I am not sure If I will go back to [his boyhood club] Newell’s Old Boys one day,” he said. “I am not sure where I will be next year. I have always said that I will end my career at Barcelona but as Cristiano [Ronaldo] said: ‘Only God knows the future.’ Things in football can change overnight.”

The Guardian believes that the huge finances involved in the deal -- Messi has a release clause of 250 million euros (£196 million) and is the highest paid player in the world -- means United and Real Madrid are the only clubs that could afford to take the 27-year-old away from Barcelona. Chelsea, Manchester City and Paris Saint-Germain all likely have the means, although would struggle to comply with Financial Fair Play rules.  

Still, the likelihood remains that Messi will remain at Barcelona into next season and, even if he does decide to leave, United, currently out of the Champions League, will have much work to do to convince the forward that a move to Old Trafford represents his best option.

While not on the quite on the same scale as Messi, Manchester United have also been linked with a bumper deal for Juventus’ Paul Pogba. Reports earlier this week claimed that United were considering a £70 million transfer to re-sign the midfielder who walked out of Old Trafford on a free transfer in 2012. Pogba has been an increasingly influential figure and driving force for the Italian champions, who understandably appear to have no desire to let him leave.

“What was a promising player has become a reality,” general director Giuseppe Marotta told Sky Italia. “Every European club is watching him and have been in contact with us. We, however, want to have a strong team made up of champions. We have an important player in Pogba and we don't have a need to sell him.”

Pogba only recently signed a new deal tying him to Juventus until 2019, however Juventus have previously suggested that they wouldn’t be able to refuse a massive offer.

Neither Messi nor Pogba, though, are realistic targets for this month. One player that is, according to the Manchester Evening News, is Everton right-back Seamus Coleman. The Republic of Ireland international has been strongly linked in recent months, and it is easy to see why. Coleman has been one of the Premier League’s best attacking full-backs in recent seasons and would offer Van Gaal the option to play him in a back four or as a wing-back, while right now he would be an upgrade on the again injured Rafael. The story states that United are confident of arranging a £25 million transfer, with it possible that it could be pushed through in January.

Coleman is not the only right-back linked with a transfer to Manchester United, however. The Daily Mirror has a fresh rumor that it is Southampton’s Nathaniel Clyne who Van Gaal wants at Old Trafford. The 23-year-old has previously revealed that he was offered the chance to move to United in 2012, but chose to go to Southampton because of the greater opportunity for regular first-team action. Now an England international, United are said to be keen to finally get their man. Convincing a Southampton side who currently lie above United in the Premier League table, though, would be far from straightforward.