Greenwood, who can also operate on the wing, started at number nine in the Europa League quarter-final win over Granada on Thursday.
Scholes told BT Sport: "He's a brilliant player, very young still so he's inconsistent at times. Again, United's forwards, they all seem to be the same. We don't know if they're a wide player or a number nine.
"Obviously, with Cavani, there's more experience but the legs are going a bit.
"They say this lad should be the one out of three that is the number nine and I think he's the best finisher at the club, but when he's in those wide areas, he doesn't get that many chances which is a problem.
"In these next few games, possibly, he's got the chance to make the number nine position his own."