Klef is a top pairing Dman (a #2) but I think the record with and without him is more of an indictment on the defensive depth of the organization than praise of Klefbom as a player.
My issue with Klefbom is that his play is a little too inconsistent for a top pairing Dman and he's almost always injured or playing injured. Injured this season, played injured (apparently) all of last season, injured in '15-'16, little nagging injuries the season before that, a shoulder injury in one of his SEL seasons. It's a long list and a very concerning pattern.
Personally I believe he's a #1, a low-end one, but still a #1 and a top30 D in this league. He's not elite in that role, but he gets the job done quite admirably and as we've seen this season, while playing some heavy, heavy minutes to boot.
The difference in record obviously doesn't tell the whole story as more goes into that than just Klefbom, but I think it's still a pretty good indicator of his importance to the team. This isn't the first time the Oilers have gone from being a competitive team to a complete nosedive once he got injured. Same happened in 15/16 and it has generally been the trend.
I actually disagree that he's inconsistent. I think he has been very consistent the last couple of years. He had a small dip in performance in a stretch this year but I think every single D (or player for that matter) in this league has those stretches over the course of a season. It's not something unique to Klefbom. But I think it becomes more eye-popping with Klefbom just because the Oilers isn't a very good team. If you have a good and deep team, then other players can make up for it when a top player is struggling. For instance, Vlasic has been garbage by his standards this year for the Sharks, but they have Burns and Karlsson to cover it up.
I don't know what you can say about the injuries either. The shoulder injury he had for most of last year reportedly was from an incident at the end of the previous season IIRC that was never fixed and it seemed like it just got aggravated during pre-season. By then he just wanted help his team (probably knowing just how important he is to this team) and kept playing through it, which proved to be a mistake.
Then the injury he got now was just unlucky, getting hit with a hard shot on his glove, giving him fractures on his fingers. Like that has nothing to do with him being prone to injuries IMO as that could and does happen to players on every team. Just pure bad luck, like the staph infection.