The former Barcelona manager, now at the helm of the Oranje, has hailed the teenager as a game-changer, drawing comparisons to the legendary Lionel Messi, whom he once coached.
"It's a stroke of luck for any manager to have players like that," Koeman said during his pre-match press conference. "With his quality, he can turn matches on their head. I was privileged to work with Messi, which was massive.
"I hope Lamine Yamal can one day reach the kind of importance Messi had for that club."
Yamal may have made his Barcelona debut under Xavi Hernandez, but Koeman's time at the Camp Nou saw several youngsters break into the first team.
Pedri Gonzalez, Gavi Pajez, and Alejandro Balde all stepped up under his watch. Pedri and Gavi have become untouchable for current boss Hansi Flick, while Balde, recently back from a serious injury, has clicked well with the German tactician.
"I'm chuffed to bits to have given those lads their debuts," Koeman admitted. "But the credit's all theirs, they've shown they're top footballers. It's great to see them doing so well, and I've got a soft spot for them. That's why tomorrow's match feels extra special for me."
The Dutchman's comments come as Spain prepare to take on a Netherlands side looking to assert themselves in the Nations League. Yamal, still only 17, has already caught the eye this season, and Koeman's words will only fuel the hype around the winger.