This is a huge issue and I'm surprised there isn't more information yet.
We've all had ankle sprains and we all know that they can be relatively mild to quite severe.
Speculating, Perk's pronouncement during the game (as reported by Doc) that he felt to be at "60%" doesn't sound like a good omen to me. My experience has been that with a mild sprain you can wrap it up and perform close to 100% right afterward (what happens the next day and thereafter is more uncertain). If Perk was saying he was at 60% (and knowing athletes' tendency to overestimate their own status), it worries me that this injury may be pretty bad.
Yes, PJ played well, but he can only be expected to give 20 minutes (tops). And Perk has the potential to provide a dimension that could really hurt LA. His 'D' is key, his size is unmatched by LA and if he were to maintain his 'monster' attitude in the paint, he'd be key on the boards. The fact that he also might have a double digit points game in him just adds to how important he could be. I'd hate to lose him for this series, but unlike Pierce who can still be effective at 80%, Perk at 80% is probably better off on the bench.