The brief history of Rays starting pitching has an interesting twist to it: No starter has had two decent seasons for the Rays; Victor Zambrano was close but was traded at the deadline last year and can't count. You can't dismiss this as just a bad team never having good pitchers though because there have been good individual performances throughout the team's existence. With Casey Fossum putting together a good season so far, mostly as a starter, I want to compare him to past Rays that have been successful out of the rotation.
Fossum suffers from small sample size issues, only 64 IP as a starter, but he was about the same quality in his 20 relief innings so I'm cautiously confident that, health willing, he'll continue on roughly the same pace.
Here is Fossum's line so far and the best starting pitching performances in Rays history, ranked by ERA+. For those unfamiliar with ERA+, it's a stat that takes a player's ERA and compares it to the league average, adjusted for the player's home park. 100 is league average, above 100 means a player is better than average.
Despite the third lowest ERA Fossum is last in ERA+. That's because the Trop has become more of a pitcher's park over the years and the average AL ERA isn't very high this season. Looking at his peripherals though he stacks up well.
3rd in K/9
3rd in BB/9
3rd in K/BB ratio
1st in HR/9
Some reasons why Fossum isn't preventing runs as well as the other pitchers:
1. The Rays defense hasn't done a very good job this season, only a .680 defensive efficiency. 2000 wasn't much better though and 1999 was far worse (.662).
2. Some of the pitchers were bloody lucky and should have had higher ERAs.
3. Fossum has hit 11 batters so far (as a starter), well ahead of even Zambrano's 2003 pace. That hurts the advantage he has in walk rate.
4. Fossum tends to be somewhat inconsistent and leaves balls flat in the strike zone. The balls don't get crushed but it does result in hard hit singles and gappers.
I would say that Fossum is middle of the pack in comparison to the better starting pitchers in Rays history. He's definitely not on top but I think he's better than some of the pitchers that beat him in ERA+. Pretty good return for nothing more than Jose Cruz Jr.