Not that he couldn't help them, it's just that he's not the best fit for the team's offense right now.
The problems with OKC aren't things that can be fixed as easily as swapping out a player like Westbrook for a player like Rondo.
This is all that needs to be said, really. This goes right back to my original point: if the Thunder are bounced out of the playoffs early, the guy who most needs to be concerned is Scott Brooks, not Russell Westbrook. The system OKC runs is one that necessitates having top-heavy scoring out of its two best players; there's not enough requisite, reliable depth for them to play otherwise. That part is on Scott Brooks; he's a great player's coach and good at managing personalities, but he's shown a weakness with unimaginative offensive schemes and an over-reliance on veterans (which is why Lamb and Jones supposedly aren't developing).
There's also the fact that the personnel OKC has also requires that Durant and Westbrook take the lion's share of scoring; that part is on Sam Presti and the OKC front office.
As for Perkins taking a step back: really, we're going to put that on Westbrook? You realize Perkins posted the worst playoff PER in NBA history last season - you know, the same playoffs where Westbrook only played two of the 11 games OKC played that postseason. Perkins took a step back because he's declining as a player, much as it might hurt those of you who fondly remember his Celtics days to realize. Westbrook had nothing to do with that and the fact that he was worse in the playoffs without Westbrook should probably indicate as much.
I appreciate the kind words from CoachBo and D.o.s., but make no mistake - I'm not discrediting Rondo at all. I just really hate this notion that because Rondo's a more traditional point guard, that inherently makes him better than Westbrook 'cause assists, man. Westbrook is great for OKC because being that he's such a credible scoring threat, he draws attention away from the less offensively-skilled players on the roster (think a Sefolosha, or a Lamb, or a Collison) and allows them to play under much less defensive pressure. Just as one can argue that Lamb could be helped with a pass-first point guard who gets him more looks, I can easily counter than Lamb is equally helped with a shoot-first point guard who gets him fewer, but more open looks.