+/- isn't an individual stat, it's a summary of what happened during a stretch of time. It's the start of an investigation, not an answer, but it's also not meaningless. Basketball is an organic sport where each of the 10 players on the court impacts the others; it's not a series of one-on-one matchups. A guy can shoot poorly while the team still operates well with him on the court in one-off situations. The game is complicated.
What +/- tells us is Creighton was objectively better with Miller on the court than off the last two games. Is that simply a result of shot variance in Creighton's favor when he was out there compared to when he wasn't? Or does it have to do with the way teams are playing on both ends when he's out there compared to other options? Either way, it isn't likely to hold up long-term with his current level of play, and the longer this goes on the more clear it becomes that he probably just isn't going to find his shot this year. But if you're focusing on Miller as the reason for the small margin of victory and the loss in the last two games, you're just barking up the wrong tree.
At this point fans are so hyper-fixated on every negative play he makes to the point of ignoring any positive results that may happen while he's on the floor. Mac highlighted the impact of his floor-spacing in the St. John's game, and while teams targeted him on defense in important situations earlier in the season to their benefit, I don't remember instances of that happening the last couple of games (could be wrong there; going off the dome). Five rebounds, three points, one assist and one dead-ball turnover in 15 minutes is an acceptable contribution for a 7th man in a game like this, even if he didn't make a 3.
Say there is pressure to keep Miller involved in the rotation going into a game. Once the bullets are flying, Mac's still playing to win and he's going to make the calls he thinks will give them the best chance of doing that. What he saw on Friday was Žugić unprepared for the on-ball pressure Marquette was applying (the happy feet travel and the pick-six pass, the latter of which you simply can't afford in that game) while Creighton was winning the minutes with Miller out there, so he let it ride. I can't really blame him for that. Marquette outscored Creighton 9-4 in the 91 seconds Zugic was on the court, scoring five points off his two turnovers (and CU's first point was Miller's second FT on a foul that happened before Fedor checked in).
Moving forward, for this team to approach its ceiling, that equation needs to start shifting in Žugić's favor. I don't know how many times Mac has told fans it's going to be a process. I can't really explain it any more clearly than I have on multiple platforms.