The Dutchman says United need De Gea at the top of his game for Monday's derby.
He told the Daily Mail: "Everyone half-expected David would have a difficult two or three months adjusting to the Premier League and so it proved.
"Spain is one of the big leagues but in England you need a little bit more.
"It took me a while when I went to Fulham at 31 and it's not strange that it took a 21-year-old time to adjust.
"I spoke to Eric Steele (United's goalkeeping coach) a couple of times and I know they worked hard on the training ground.
"De Gea looks similar to me. I was quite skinny when I started out. He is confident with his feet also and that's a big part of being a goalkeeper nowadays.
"The aerial battles he just has to adjust to.
"Maybe he's going to be the best in two years' time but you need to have him now to make him the best."