Manchester United have all but secured their primary objective this season of a return to the Champions League, but Louis van Gaal’s squad rebuilding is set to continue in earnest during this summer’s transfer window. Some much-improved performances over the past six weeks have seen the Old Trafford club sit comfortably in third place in the Premier League -- good enough for an automatic place in the group stage of next season’s Champions League. Yet a 1-0 defeat to Chelsea last Saturday, when United had several players missing through injury, ended their slim title hopes and provided a glimpse at the improvements still needed to return to the very top.

Van Gaal was missing two of his center-backs at Stamford Bridge, and has been hampered by disruptions to his defense throughout the season. And, although he has recently signed Chris Smalling to a new long-term contract, the heart of the back line looks set to be a key area of focus for the club’s business this summer.

Reports continue to suggest that Borussia Dortmund’s Mats Hummels is the leading target. The German World Cup winner has revealed in recent weeks that he will consider his options in the summer, and the departure of coach Jurgen Klopp has cast further doubt over the future of Dortmund’s captain. On Wednesday German tabloid Bild claimed that United had already made a bid of 30 million euros (£21.6 million) for the 26-year-old.

For a player of Hummels’ caliber and reputation, a fee of that size is highly unlikely to be enough to satisfy Dortmund’s demands. Indeed, Bild states that a bid closer to 40 million euros (£28.7 million) will be required. Dortmund sporting director Michael Zorc has responded to Bild’s claims of a Unite bid by telling the publication: “I can’t confirm that. I have no desire to comment repeatedly on this topic. I’ve already given my thoughts a few weeks ago.”

Hummels is not the only central defender linked with a transfer to Manchester United, however. Spanish publication Mundo Deportivo has stated that United are also one of several clubs with an interest in Atletico Madrid’s Miranda. The Brazilian has formed a formidable partnership with Diego Godin, who was also said to be interesting United in January. But Mundo Deportivo believes Miranda could be allowed to leave this summer, although Chelsea, and, according to Italy’s Gazzetta dello Sport, Juventus are also interested. With Miranda set to turn 31 at the start of next season, United may be hesitant to pay a large fee for his services.

Speculation suggests that Hummels may not even be the only Borussia Dortmund player Manchester United are pursuing. Michael Carrick has been one of the team’s most influential players this season, and his absence was sorely felt against Chelsea. But with the midfielder now aged 33 and having been bothered by several injury complaints this season, the Daily Mail has reported that Ilkay Gundogan is being lined up as a replacement.

Gundogan was superb in Dortmund’s run to the 2013 Champions League final before missing more than a year through injury. But now getting back to his best, his future is back in the spotlight. Last April the 24-year-old signed a contract extension, but one that only tied him to the club until the summer of 2016. Still, with Daley Blind also able to play the holding midfield role, United may decide that their transfer priorities lie elsewhere.

And it may well be another target who becomes the first arrival of the summer transfer window. Memphis Depay is one of Europe’s rising stars after scoring a league-leading 20 goals to fire PSV Eindhoven to the Eredivisie title. His club have already publicly resigned themselves to losing the 21-year-old winger this summer, with a host of clubs thought to be on his tail. But the Manchester Evening News has stated that United are optimistic of getting a deal done, with Van Gaal’s relationship with the player a key selling point. As coach of the Netherlands, Van Gaal selected Depay in his 2014 World Cup squad.