He's 3rd on their depth chart right now.
And Marquis and Richards were a head of him before they got hurt. That makes him a 4th or 5th starter. What's difficult to understand about that? Also, having front of the rotation stuff and being healthy doesn't make him a front of the rotation starter. He's 9-8 with a 3.40 ERA pitching in arguably the best pitching park in the league. No matter how you try and slice it, that's not a front of the rotation starter.
Is it possible that he's just been set back by injuries? Maybe but this isn't a case like say Strasberg where he went out in his rookie year was 15-6 with a 3.16 ERA and 11 k/9 and then hurt his elbow. Nothing Cashner has done thus far screams elite pitcher. What he's done this year statistically hasn't been much better than what Wood has done and I don't see many suggesting Wood will be a front line starter. Additionally, he has 3 years of arbitration left starting in 2014. So, if he does figure it out in the next 2 years they only have him cost controlled shortly. To project him as a front line starter at this point is pure speculation. Just because he throws hard doesn't guarantee anything. There's tons of guys who have a similar make up as him and were forced into the bullpen.
Also, if there's one thing this front office has been exceedingly good at, it is pitchers. We're talking Maholm, Feldman, Strop, and Wood have all been much better than expected. Arrieta has even shown flashes. Jackson is questionable but his metrics like him better than his regular stats.
Rizzo's floor appears to be what it is now. He may never reach his ceiling but at the very least he's proven to be a solid major league player who can start some where at 1B in the league no matter what. Cashner's best, and only, year he's put up numbers consistent with 4th and 5th starters while pitching much better in arguably the best pitching park in the league. He's had significant injury history and he's 27. So, to sit here and suggest he's even a 25% shot of becoming a top of the rotation starter is a stretch. If it were just a matter of him being healthy he'd be putting up better numbers this year. There's more than enough reason to be skeptical about Cashner.