Manchester United are set for a hectic summer in the transfer market, even after making an early splash by bringing Dutch starlet Memphis Depay to Old Trafford. It’s thought that the club intends to spend big in order to further rejuvenate a squad that will be back in the Champions League next season, and looking to seriously challenge for the Premier League title.

One area manager Louis van Gaal is most eager to strengthen is the center of his defense. Although Phil Jones and Chris Smalling showed signs of progress last season, United have yet to adequately replace the experience and top-level quality of Nemanja Vidic and Rio Ferdinand since the paid departed last summer.

The long-time primary target to fill that position has been Mats Hummels. However, United continue to face a battle to prize the World Cup winner away from Borussia Dortmund. While a few months ago the 26-year-old suggested he was ready to leave, the tone has changed since a change of management at the Bundesliga club.

While Hummels continues to retain the focus of United’s attentions, they are exploring alternatives, including Valencia’s Nicolas Otamendi, according to ESPN. Otamendi enjoyed a superb season for a Valencia team that finished third in La Liga, but he has recently made it clear that he is keen to move on, with United thought to be his chosen destination. Valencia, though, have insisted they have no plans to make his departure easy, stressing that his 50 million euros (£35 million) buyout clause will have to be met. United have understandable hesitations on committing to such a large fee on a 27-year-old.

It has also been reported that Spanish giants Real Madrid are set to enter the race for his signature, and, intriguingly, United also continue to be keenly linked with one of the Spanish giants’ current center-backs. According to Spanish sports daily Marca, United made an inquiry about the availability of Raphael Varane during one of the meetings over the proposed transfer of David de Gea to the Bernabeu.

While regarded as one of the world’s best young center-backs, the 22-year-old has struggled to gain a regular place in the team in front of Sergio Ramos and Pepe. But Marca states that Madrid have refused to consider letting him go and plan to make the France international a first-team regular next season in order to  keep him happy.

United’s discussions with Real Madrid are also likely to have included mention of Gareth Bale’s name. During what was a difficult season in the Spanish capital, the Welsh winger was frequently cited as a target for the Old Trafford club. Yet the message out of Madrid, and indeed Bale’s camp, continues to be that he will not be leaving just two years after the 10-time European Cup winners made him the world’s most expensive player. That message has been backed up by Marca, which reports that Real Madrid have told United they won’t be selling.

A more realistic transfer target at this point is Monaco’s Fabinho. Manchester United are on the lookout for a right-back after winger Antonio Valencia spent much of last season filling in out of position. While United have been strongly linked with Southampton’s Nathaniel Clyne, there are now strong reports from French sports daily L’Equipe that they are keenly pursuing Monaco’s Fabinho. The 21-year-old, who made his debut for Brazil ahead of the Copa America, has only just had his move to Monaco made permanent for 6 million euros (£4.3 million) after two years on loan in the principality. But now United are said to be willing to pay 14 million euros (£10 million) to take him to Old Trafford.