I think we can rule out concussions. He's been attending all off-ice team events, traveled to D.C., he's functioning like someone who doesn't have a concussion. Concussed players usually don't partake in travel and such. It's his knee. Seemed like Mitchell postponed surgery and the knee got worse and as a result, he may have damaged it further (could be torn ligaments, who knows).
In somewhat positive news, from the LA Times article, Mitchell's agent indicated that he could be back by next season. His status is up in the air as to whether or not he'll play again in 2013, but we don't know how he'll recuperate from his latest surgery. The Kings just have to move on and if Mitchell ever does come back, it's a bonus. But if they do lose him, it is a significant blow to the team and you would hope that Regehr would do an adequate job in replacing him.