Needs to fill out, and learn to play with his back to goal(like Martial did). And needs to improve his heading ability(like Rooney did). If he can do these two things, he will become an elite RVP-esque CF.
It sounds like you think he needs to be a completely different player to who he actually is...
Not sure why he needs to fill out...he has bulked up already and I don’t think he gets knocked off the ball too easily...his ball retention is better than any of our frontline and bulking up is not always a good thing. His change of pace and ability to go either way is not dependent on putting on muscle.
The back to goal thing, Martial is an odd example...he is quite shit with his back to goal...he can do it but not to any great effect. Arguably, the attempt to transition Martial to a more typical number 9 has undermined and taken away from the qualities he had, pace, close control, dribbling and finishing. (He can’t finish when he is facing the other way!) Mason is never gonna be a foil type striker and so I would think it’s more important he plays to his strengths and gets turned and running at defenders going left or right...
And as for heading, yes he needs to improve his heading but again, Rooney is an odd comparison, he wasn’t a prolific header of the ball and Rooney said himself he hated his season as an out and out 9 as he just didn’t get enough of the ball. I see Mason more in that mould. He adds more value on the ball with his control, ability to beat a man and the fact that he is one of the few genuine threats we have from distance (Bruno maybe).
He has been injured and benched in the last half a year. Just play the kid...he needs to play with clear instructions...RR talks about being proactive...In time...Greenwood will be a player that helps us dominate games from the front...He is a plan A player...let’s buy another guy to be plan B...back to goal...someone we can target with crosses.