"It will be two big games for Forster," the Italian told the Scottish Sun. "We can see he has class and talent, but the Champions League knockout stages are where your nerves are tested.
"When you are nervous then you are more likely to make a mistake as a goalkeeper. I like what I have seen of him so far; he can go on and become one of the top five goalkeepers in Europe.
"Joe Hart is a very special keeper, but if Forster keeps up this form he will be a real rival for him at international level. He is good enough to be the first choice at the World Cup."
Forster has been at Celtic since 2010.