Grant joined in the summer from Stoke City.
Alvarez told manutd.com: “To be honest, I do not agree that Lee is only learning from us. Since Granty came here, he has been a really, really, really nice surprise. We are learning from him.
"We are really learning from him, because he is a really, really good goalkeeper, a really, really good professional, and as a man, the way he interacts with the rest of the colleagues, he is unbelievable. We are really lucky to have him.
“We have to be sure to say we have of the three best goalkeepers in the world, because each of them has really good skills. For example, Sergio is an experienced goalkeeper in the Argentina national team so what more can I say about him? It is a pleasure to work with all three goalkeepers."