Speaking on his YouTube channel, the 45-year-old said: "I watched him at Ajax and he was brilliant in that year when Erik ten Hag was the manager, and they got to the semi-final [of the Champions League] and got beat by Spurs. He was brilliant, commanding, everything you want in a defender.
"He's had a lot of injuries, hasn't started many games over the last two or three years and Man United are spending £45-50 million on him. Is he going to come into Man United and make a difference? I think it's a big ask for him and all eyes are going to be on him. Big pressure, big opportunity for him, but there's a lot of uncertainty I think with this signing because of his previous seasons with two big clubs (Juventus and Bayern Munich) not going as well as we'd have anticipated."
He added: "I don't think he's the answer, but it's a great opportunity for him to go in there and go 'You know what guys? I'm going to rectify my last couple of seasons. That's not the real me, and I'm going to show you."