Bunney came off the bench to head the only goal of the game 12 minutes from time with his first touch.
It ended a stubborn resistance from Plymouth, who battled gamely after Maxime Blanchard's early red card for a professional foul on George Donnelly.
"Joe has talent and he will only get better," Hill said.
"It's his first taste of real men's football, he has great movement, good appetite and he knows where the goal is.
"He may have a bad game but he'll put the ball in the back of the net."