When I heard that Garnett was a late scratch against the Bobcats, I figured if there was an injury involved, it was a minor one and Doc was using it as a good excuse to give him some much needed rest against an opponent that the Celtics could (especially properly motivated) be able to handle without him. That's probably (in general) what Doc was thinking, but it sounds like he might miss the Heat game as well.

Doc Rivers said Garnett is '50/50 at best' to play against Miami -Celtics blog - Boston Globe basketball news

Celtics center Kevin Garnett (left adductor strain) did not practice Sunday here and coach Doc Rivers said Garnett is "50/50 at best" to play Monday against Miami. Garnett was a late scratch Saturday before the Celtics' 105-88 win against Charlotte with the injury, which is located in the thigh area. Rivers stressed Sunday that he doesn't believe the injury is that serious and he said it ranked as a "1" on a scale of 1 to 10.

Celtics Insider | Boston Herald

"I don’t think it's that serious," said Rivers, "but it's just at that point in the year where you’re just not going to play him if he's not feeling great."

So there's a legit injury, but it isn't serious. Yes, the Heat are a more interesting matchup, but at the end of the day it is just one more game on the schedule. If it was the playoffs it would be another thing. But for now, keeping everyone healthy and fresh is more important than a statement game.

With that said, if KG wants to go, I think he'll be out there. Also, with or without Garnett, I think we'll certainly see the Celtics "up" for this game.