Manchester United on Wednesday announced they had completed a deal for the Netherlands international worth a reported initial 39 million euros ($46 million).
Having joined Ajax at the age of 11, Van de Beek scored 41 goals and provided 34 assists in 175 appearances in all competitions and helped them win the Dutch league title in 2018-19.
"Dear Manchester United fans, I hope you’re doing well," Van der Sar wrote.
"It seems our paths have crossed again. One of ours is joining you this season. And like so many players before him, he’s been with us since he was just a little boy.
"Shortly after his debut, he became one of our best. Especially the last couple of years were amazing. From the Europa League final (no hard feelings) to our Champions League run and winning the Dutch championship.
"The team he was part of showed the world who we are and what Ajax stand for.
"Like you, we pride ourselves on being one of the best at developing talent and giving young players a shot at the highest level. You could say your new star is the embodiment of that pride."
"That’s one of the many reasons we don’t like to see him leave, but we understand it’s time for him to move on. To dream on.
"And where better to do so than in your theatre. Trust me, I know. Please take good care of Donny, and help him dream.
Donny van de Beek has said he will wear the number 34 on his jersey as a tribute to former Ajax Amsterdam team mate Abdelhak Nouri, who suffered brain damage after a heart attack in 2017.
Nouri, who also wore the number, made his senior debut in 2016-17 before his playing career ended prematurely after he suffered cardiac arrest in a pre-season friendly in Austria.
Despite being revived on the pitch and rushed to a hospital, Nouri suffered serious and permanent brain damage.
LIVE TV
