Judging by the speed of the collision and Paajarvi's inability to put weight on his knee, the prognosis isn't going to be a good one and it's likely Paajarvi is going to miss significant time.
Will Hartnell get a call from Brendan Shanahan about the hit? The hit isn't as clear-cut as the other incidents involving the Oilers this season, and there certainly wasn't any head contact, so Shanahan has gray area to sort out. It's clear that Hartnell was lining Paajarvi up for a big hit, but Paajarvi recognized it and contorted his body in order to avoid the hit. Hartnell ends up with nothing to hit but Paajarvi's knee and wrecks it. Hartnell did nothing to avoid that contact but he was moving fast, possibly too fast to avoid the hit anyway.
I suspect Shanahan will issue a fine with no suspension and leave it at that.
The resulting injury is a terrible one. Paajarvi's performance since his call-up was reminiscent of his play to close the 2010-11 season - he's skating hard, playing in both zones and using superior size and speed to win battles.
If he is out for a significant amount of time, it's now a sure thing that Linus Omark will get a phone call, especially with Ales Hemsky's time in Edmonton down to a single game.
Update: The NHL announced that there will be no supplemental discipline for Scott Hartnell.