I'd have Mahinmi play in addition to Blair. Since they don't play the same position, they will not be getting each other's minutes.
When Dice is injured and getting zero minutes, that's exactly where Mahinmi should be getting minutes. But then again it must be all those mystical in-season practices fans seem to think the Spurs have.
Again, Bonner's not a five by any stretch of the imagination. Mahinmi can't do what a three point shooting role player does. Fine. Mahinmi shouldn't be coming in when you need a big three point shooter. He should only be on the floor when you need a seven foot guy who can play defense and rebound, things Bonner just can't do.
Mahinmi could be anything, including as good as he's looked during the small number of minutes he's gotten, or he could be really bad, which would still make him a better center in defensive situations than Richard Jefferson. It likely doesn't matter as the Spurs are going to be matched up with a much taller team in the playoffs and clearly show a commitment to countering size with lack of size.