Lemar signed for Atletico in the summer from AS Monaco.
Griezmann told L'Equipe: "He is 22 and what he does at his age is tough. We need a good last pass, it's our weak point, he'll bring us that.
"He started as a starter, he is already an important player. It is also up to us to help him, especially when he is a little less inside, which is normal during a period of adaptation.
"He may have a dip in October, for example. It's up to us to be there. He's in good hands."