This is the first specific detail we've heard about Pierce's injury. The MCL is pretty important to your lateral movement, and there were plenty of times after Pierce tweaked it again in Game 5 that it was obvious Pierce wasn't moving from side to side very well.
But he went through shoot-around. I don't know if he'll be fitted for a special brace or anything to keep that in check, or if he's going to try to gut it out as is.
FYI, all 15 players participated in shoot around, so everyone appears to be a go tonight.