Lopez is much better on D.
Monroe is much better on O.
Considering the Knicks biggest weakness was D last year, and they've already won like 11 games and are playing better basketball than a more talented Bucks team, it shows that Lopez was a good investment for D purposes. There's no way the Knicks are 11-14 with Monroe, because despite his better O, he's a liability on D in that he can't defend perimeter bigs nor can he anchor inside. Lopez anchors inside and gives you a presence. Our D (which is arguably) the most improved D in the league is in large part due to a scheme designed around Lopez or Porzingis defending inside with their shotblocking/length.
The real issue longterm is Lopez/Porzingis/Melo. That trio is too slow to start or play big minutes every night. Fisher must realize he has to limit the minutes of those 3 together.