The injury occurred early in the period, when Malkin was skating hard to break up a two-on-one chance for the Detroit Red Wings. While he successfully negated the breakaway, he slid feet-first awkwardly into the endboards. Malkin attempted to get up slowly but immediately fell to the ice in pain. He had to be helped to the dressing room by a couple of teammates while putting no pressure on his left leg.
Here's the play, via The Pensblog.
It's the second time Malkin left the ice with an injury on Saturday. Malkin was shaken up after a collision with Penguins teammate Robert Bortuzzo earlier in the second period but returned to the game a few minutes later.
The only good news right now is that the injury appears to be Malkin's left leg, and not the right leg that he had surgery on in 2011. But that about does it for positives, as losing one of the league's best players for any amount of time is a huge blow for the struggling Penguins.