Ajax chief Van der Sar told MUTV: "We've seen a lot of him this season and he's probably one of the main reasons why Ajax are not champions! Although, of course, there are some games we should have won and not lost.
"He has played tremendously well for PSV and was in the World Cup squad. He had three games - he came on and did well and scored two goals. So he had a lot of confidence coming into the new season.
"PSV hadn't won the title for the last seven years - Ajax had won it for the last four seasons - but he started playing unbelievably well for them. They came to the Amsterdam Arena earlier in the season and had a couple of counter-attacks against us which really killed us. He has scored 21 goals this season and a lot of assists. He can hit an amazing free-kick as well.
"Of course he is not the finished product yet - there is still a lot to work on for him - but potentially he is a really good player and I think United have made a good deal. He's a tremendous player and in the years to come I think he'll show that. Louis is a great manager to help fulfil the potential he has."