A year ago, Van de Beek left Ajax for United in a move which hasn't panned out as hoped.
Van der Vaart said on ESPN: "If no one leaves United, he must be lucky to get in the starting lineup.
"He is far too good to sit on the bench for two seasons. He needs minutes very quickly. He is still young, but time flies.
He added: "He fit perfectly into the Ajax system, but he does not fit at United.
"When I went to the Premier League, I found out that it went much faster there and you could never rest for a while. Everyone expects you to keep going. That was the hardest thing for me. But on the other hand, there was a lot of space between being able to play the lines. For a number ten like me, that was great."