I'd be fine with Zaza Pachulia.
Has size, tough, solid all around. With Monroe maxing out, he could be attainable for a decent price. Also, instead of Kanter, I'd go for Steven Adams. We got the inside scoring and rebounding taken care of with Lee, we need a dude who is big, will play big and can protect the rim, enter the Kiwi.
Pachulia/Adams - Lee - Amir. That's not a bad three man rotation for the bigs.