Livan and the Nats are aware of Hernandez's knee problem. The surgery would fix it easy enough, but he would be out the rest of the season if he was to have it now. If its effecting his game, or if the Nats fall out of the race, he'll have it. But I think as long as he can pitch to his satisfaction and the Nats stay in the race, he will tough it out.
If his knees gets worse, he wont have a choice and the Nats will have their umpteenth player go onto the DL. The way things have been going for the Nats this season with the injuries, you breathe a sigh of relief every day no one new goes on the DL.
I don't think in the history of Baseball, a team has been this unlucky when it comes to injuries.