Edwin van der Sar says Donny van de Beek has no better place to flourish than at Manchester United and asked fans to take good care of the midfielder in an emotional open letter.
Netherlands international Van de Beek completed his move from Ajax to Old Trafford on Wednesday in a deal that could eventually reach £39.1million (€44m).
Van de Beek became an influential member of an exciting Ajax side after rising through the youth ranks to break into the first team as a teenager during the 2015-16 campaign.
The 23-year-old won Eredivisie and KNVB Beker medals with Ajax, while he was part of the team that finished runners-up to his new employers in the 2017 Europa League final and the side that reached the Champions League semi-finals in the 2018-19 campaign.
Red Devils legend Van der Sar, now CEO at Ajax, talked up Van de Beek's talents in a classy message to United supporters in the Manchester Evening News.
"Dear Manchester United fans, I hope you're doing well," the former Netherlands goalkeeper 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.
"Enjoy the future."
Van de Beek adds to a midfield that already boasts the talents of Paul Pogba and Bruno Fernandes, and he will hope to make his United debut when their Premier League campaign begins against Crystal Palace on September 19.