Since 1990, 3 starting pitchers have had a post-90 career ERA lower than 3 - Jose Rijo, Clayton Kershaw, and Pedro Martinez. When you add in his pre-1990 career, Rijo drops from the list.
Is it impossible that Detwiler can maintain an ERA under 3? It's not unprecedented, and guys can do it for a streak of years. Is it likely? no. Is it likely that's where his career will end up? You can say with a greater degree of probability than you can say with most medicial diagnoses that the answer is no.
The value of looking at the whole series of metrics, like K rate, velocity, pitch movement, BB rate, contact and other measures is you get a better approximation of a player's abilities going forward than just looking at ERA over 25 - 30 starts. No one is arguing what Det has done. The folks who are saying he will merely be excellent instead of one of the 3 best starting pitchers in recent times are arguing likelihood when projecting future performance.
