It's not really that the D is a problem, but more a season long outlook about where this team is going.
Kulak is playing top 4 minutes, a role he's only ever been able to be effective at for short burst, and Stecher is playing 13 mins a night on the bottom pairing, which is about the most you can ask for from a rather flawed player. They're fine where they are right now and aren't causing problems, but I'm not really ready to check them off as "good enough" to take a run at the cup in their current roles when the games get tough. When you get into the playoffs and start playing against closely matched opponents, weak links(Bear a few years ago, the hitch in Woodcroft's system a couple of years ago) become exploitable liabilities that make the difference in 7 game series. Having another guy in the top 4 to push everyone into their idealized lineup role would make this team better, even accounting for the fact that the Oilers are already looking like they have morphed into one of the elite defensive teams in the NHL under Knoblauch.
No disagreement on S.Skinner though, he's been bad all year. If he was even playing at a mediocre level, a bunch of the games the Oilers played this year wouldn't have even been close, yet they're dominating and still barely scraping out wins if not outright losing.