If there’s any type of fighter Penn has struggled against, it’s wrestlers. Matt Hughes and Georges St. Pierre were both bigger and stronger than Penn, but they are also both excellent wrestlers, and they both beat Penn by outworking and outlasting him. You can give him a pass for having a broken rib or bad cardio in those fights, but the blame for both of those problems rests with Penn.
A powerhouse of a wrestler is tough on a jiu-jitsu fighter because his takedown ability and takedown defense makes it difficult to gain top position on him, which means that if you can’t beat him on the feet and can’t sweep or submit him on the mat, your chances of winning a decision aren’t good.
I’m not saying Penn’s standup isn’t better than Sherk’s. It is. But here’s a fun quiz: who’s the last person Penn beat by TKO? The answer is Paul Creighton, and it was at UFC 37 in 2002.
You could argue that he beat Joe Stevenson with strikes before finishing him with a choke, but Sherk isn’t Stevenson. To beat him you have to either overpower him and be a better wrestler than he is, or you have to become the first person to knock him out or submit him.
Penn probably isn’t going to KO him, and a decision is unlikely unless he can nullify Sherk’s takedowns. His best chance is submission, and Sherk has never been submitted.
I'll agree with Fowlkes that Sherk is probably being slept on here a little. A guy as tough, tenacious and driven as Sherk is always a threat and can never be counted out. That being said, I think Fowlkes' analysis is missing the more critical elements.
First, I don't think Penn's struggled against wrestlers at all. He crushed Hughes their first time out and was doing so in the rematch until he suffered a rib injury. Second, he walked into the fight with Georges St. Pierre with half-assed cardio and arguably won. Second to Matt Serra's beating of St. Pierre, no one has damaged the French-Canadian like Penn has. Matt Lindland and Randy Couture - two elite, much larger wrestlers - have stated Penn's wrestling (and especially his takedown defense) is absolutely outstanding. Penn doesn't have the gluey top control common to many wrestlers, but that's do more to choice than limitation. And while Joe Stevenson isn't a world class wrestler, he is an exceptional talent and yet could do nothing but survive against Penn.
As for Penn's inability to TKO/KO opponents on the feet, there's no argument that he hasn't accomplished that finish in years. But in interviewing Rob Kaman, he told me personally BJ Penn is a K-1 level striker. That rare ability always leaves open the possibility that a knockout could happen at any time. I do agree with Fowlkes, though, that Sherk likely will not be knocked out in this fight. On the other hand, I expect him to absorb significant punishment as long as the fight stays standing.
And it's true Sherk has never been submitted. Sherk has very underrated jiu-jitsu and is an excellent guard passer (I doubt he'll pass Penn's guard more than a couple of times, though). But Sherk's jiu-jitsu isn't even close to Penn's, so at worst for Penn they'll cancel each other out. What I see happening, however, is Penn actually outwrestling Sherk - yes, outwrestling with top control - or finding a way to Sherk's back and even the mount. Sherk is hard to sweep and has excellent reflexes, but he doesn't have the defense to avoid Penn's uncanny ability to take the back. From there it is perfectly reasonable to suggest that even if Penn can't land a submission, he can pound out a win.
For me, it's hard to accept the idea that Sherk can win a fight on cardio alone against a fighter with many more offensive and defensive skills who will likely come with at least serviceable cardio himself. Against an in-shape Penn, I don't see how anyone can win on cardio alone.