The thing is, I don't think Monroe has ever played with a true floor-spacing big to provide him space in the paint to operate. He's always been paired with people like Drummond or Parker, though Parker is a little better for next to him.
He's a true, low-post center that could legitimately put up 20/10 in the right system. Add a floor spacing, defensive big like Horford next to him and you have one of the best 4/5 combos in the league.