The former Everton striker has shown outstanding form for United this season, scoring 25 times.
Everton are set to face United in the Premier League on Saturday.
Moyes said: “I think Wayne is a top player and I think he is going to be a great player.
“He has added goals to his game, maturity and understanding – all the things you would expect from a maturing player.
“I think time will be the test of whether he becomes the best in the world but he is certainly getting very close to it.”