Valencia will meet Barcelona, with Gaya's battle against Yamal likely to be decisive.
He said, "I don't know if he's the best, but he's one of the best. I remember seeing him on his first day with the national team when I was 16 and I was amazed.
"I said he was going to make history and I think he's going to do just that. He's young, but his potential has no limits. It's always nice to face people like him.
"I've seen Lamine quite a few times and he's very difficult to defend. You don't know if it's better to close down on him or give him space. He has the ability to get out to both sides and if you don't drive the ball he can make things difficult for you. You'll have to be aggressive and stay close. You have to make him run backwards and that could be the key."