The Holland midfielder has struggled with Juve after last summer's arrival from Atalanta.
But Van der Sar told La Gazzetta dello Sport: "I've known Koop since his time at AZ Alkmaar, we even followed him for Ajax. In the end we had no space in midfield and Atalanta bought him. Koopmeiners is important for Holland and I'm sure he will be for Juve too.
"Maybe the Dutch atmosphere of PSV will unlock him and a new season will begin for him. He's a complete number 8, I like him a lot. He needs to take some pressure off, which is part of the game."
On Michele di Gregorio, he continued: "I've only seen him in a few games and he seems like a good goalkeeper. If Juve, after (Gigi) Buffon and (Wojciech) Szczesny, chose him, well there must be a reason.
"However... My favourite Juve player is (Francisco) Conceiçao. He's a great talent, Francisco, and he has a top mentality: he doesn't hold back and is very determined.
"I was his director at Ajax in 2022-23 and he is a really good guy. I am happy for Francisco and for Juve: a great move. And maybe he will be the decisive man in the double challenge against PSV with his mix of feints and dribbling. Conceiçao only needs two of his bursts to make the difference."