The German great also questioned the quality of the players within the squad.
"It is the way that you play, what is your strategy," he told BBC Sport.
"If you have Cristiano Ronaldo and he takes his breaks from defending, but if he scores you goals in the box, that's what he does and what you have to know.
"He knew exactly what the players needed to perform and they have a strategy and a DNA. They have more in their game.
"It is still not 100 per cent clear to me if Manchester United are going to be successful or not. They have great names like Paul Pogba and Cristiano Ronaldo, and I think that Marcus Rashford will help them a lot in the future because he has been injured. It has not been easy for them.
"But if you compare their football to other teams like Chelsea, Manchester City, Liverpool or even like Bayern Munich it is not on the same level at the moment. United can always hurt you in one game, that is clear to me, especially at Old Trafford."
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