My guess is still his defense. When he came in the second half, Bohannon ran him off two screen actions. One for a score and one for a good look. (From what I recall) I specifically remember MAAR came in for him after a TV timeout to guard Bohannon and played the screen action differently than Simmons.
Matthews aside, I do agree Simmons had too short of a leash last night. Felt like he earned more opportunity with that first half. At least let him play through those (maybe?) mistakes. I still feel like Beilein is waiting for him to buy into the defense before leaving him out there longer.