The former Chelsea and Netherlands star claims he was surprised van Gaal only added one centre-back to his team given Rio Ferdinand and Nemanja Vidic had left the club before he arrived.
And as it has turned out, all three have suffered constantly from injuries this season, leading to regular appearances for the likes of Patrick McNair and Tyler Blackett
Speaking about United’s defence, Gullit said: “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.
“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.
“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.”
Given Borussia Dortmund’s perilous Bundesliga position over Christmas, Hummels was once again linked with a move to Old Trafford in January.
With United still struggling at the back, Rojo has been the most solid of the centre-backs to date and it’s assumed several more defenders will join him if van Gaal loses patience with the likes of Jones, Evans and Smalling.