McGrath has been stunned by the former Fulham defender's progress.
He told talkSPORT: "Smalling, for me, is amazing.
"Especially when he takes off with the ball he seems to move so quickly and so easily up the field.
"His reading of the game is brilliant and he is so cool on the ball as well, he just doesn't seem to have a nerve in his body.
"But then there are a few young players like this. [Sir Alex Ferguson] seems to bring these players in and they just perform. Training with the older players like Rio [Ferdinand] they get used to playing with top players and become top players themselves."