I think Monroe is a good fit for Stevens system. He is a very good passer and a smart player. He plays with good quickness in transition and in the halfcourt. He gives the team a true low-post option. Basically everything Tyler Zeller does plus shot-creation, improved passing and superior rebounding.
While I would like to see the Celtics acquire a true low post option, I'm not sure I agree about Monroe's fit as that sort of option.
The biggest difference between Monroe and Zeller is that Tyler at least has the threat of a jumper from beyond 10 feet. Monroe has never shown an ability to reliably hit a shot from outside of the paint. Zeller can also move very quickly for a seven footer; few guys his size can slice down the lane and finish as he does.
That means Zeller can be a cog in the team's ball movement, setting picks and catching the ball near the three point line, and still present the threat of knifing down an open lane to the basket, catching, and finishing the moment there's an opportunity.
Although I think a post option can theoretically fit into Stevens' pace-and-space system, you'd need a guy who can also perform other roles in the offense, and who can get into position and get a decent low post shot off without eating up a bunch of the clock. I may be selling Monroe short here, but he doesn't strike me as that kind of player.
If Monroe had a reliable free throw line / elbow jumper, he could operate out of the high post without killing spacing too badly. He'd need to improve his shot a lot, though.