That's the thing about Kovalchuk. He needs space to be effective. The Rangers with their passive system gave it to him. The Kings are not doing it. I think he's hurt but it's not possible to extrapolate and say he'd be lethal in this series at 100% health. Maybe on the PP, but even then he's not getting much time and space from the Kings. They are taking him out of the game and I am not convinced they wouldn't be taking a 100% Kovalchuk out of the game.
Kovalchuk is able to buy space when he is healthy. Right now he is not trying to beat anyone 1 on 1 because he just can't make the skating/stickhandling moves necessary to do so. When healthy, he's a terror in the neutral zone with the puck - that burst isn't there, he's not able to back defenders off, and as a result he's probably dumped the puck in more during these playoffs than in the rest of his career.
He still only has 2 ES goals in the playoffs.