Former Chelsea star Ruud Gullit has revealed his compatriot Louis van Gaal was "crazy" about Borussia Dortmund centre-back Mats Hummels and "wanted to sign" the Germany international during the summer transfer window.

Following David Moyes's disappointing campaign at United, the Dutch manager spent more than £150m to overhaul the squad with Ander Herrera, Luke Shaw, Marcos Rojo, Angel Di Maria, Daley Blind and on-loan Radamel Falcao.

Even though Rio Ferdinand and Nemanja Vidic left Old Trafford to join Queens Park Rangers and Inter Milan respectively, Van Gaal only signed versatile defender Rojo and failed to buy a new leader for his backline.

Reports in the summer of 2014 claimed the United manager tried to sign Joao Miranda and Hummels but both Atletico Madrid and Borussia Dortmund refused to negotiate their departures.

Now Gullit has confirmed Van Gaal wanted to sign the Germany international and admitted he was surprised by the United manager's final decision not to sign any "good" centre-backs.

"The only thing for me that is a surprise is Van Gaal didn't buy any good central defenders - that had to be his priority," Gullit said to Goal.

"He was crazy about Hummels and wanted to buy him. So I would have bought Man United some good defenders because that was the weakest link."

Nevertheless, the former AC Milan midfielder has backed Van Gaal, who has managed to cope without a top centre-back as the team continues to get decent results.

He said: "For me, that they did not was a surprise. Nevertheless, he gets results. The more results he gets, the more confidence they get and the better they will play."

Meanwhile, when asked about Chelsea manager Jose Mourinho's policy of selling young players such as Romelu Lukaku and Kevin De Bruyne, he said: "It's their way to make something out of their youth system because they sell them.

"For me personally, even though I'm a Chelsea fan, I saw some Chelsea players there who never played - they only played second-team games. I don't want that for a youngster. I want him to play in a league every week where we can play and show himself - that is the problem with that. I advise players when they're young in Holland to stay there as long as they can."