Theis's loss severely weakens the team defense. Monroe is a much better offensive player but he just can't move defensively because he's not that great an athlete (by NBA standards). Theis is cat quick, plays great team defense and can rim protect. Couple his loss with Smart going down and they're going to struggle against good teams.
It'll show up in their inability to get stops at key moments of games.
The stats simply don't bear this out. Monroe has an excellent DRtg and DBPM. Where is this insane fallacy coming from that he's a bad defender?
You must be looking at some of the garbage stats found on basketball reference. Monroe gets a good defensive rating because their formula is derived from defensive rebounds, steals and blocked shots. Because he has a high def. reb %, steal % and block % he is awarded a very good defensive rating, you simply cannot determine a players defensive value by looking at those few box score statistics. The C's have given up a whopping 110.7 points per 100 possessions with him on the floor so far. He is terrible defending on the perimeter, extremely slow and is targeted relentlessly when he is in the game. These garbage formulas think Andre Drummond is good also and that is just ludicrous as his team is always significantly better defensively without him on the floor.
Monroe's main weakness on D is his slow feet guarding the pick and roll.
He's a rotating door defending the PnR unfortunately.
I think Brad will clean his stuff up a bit, but his lateral footwork is just awful.
A shame because he has good footwork, just not sideways footwork.