In truth, we should have expected this. The Twins started the year with five young pitchers in the rotation. None of them had more than one full season of major league success. And there were really no pitchers ready immediately behind them. We saw that when R.A. Dickey stepped up to take a start early in the year. But while some setbacks were expected, three of the "young five" are now on the DL and the performance of the rotation does a lot to explain the Twins struggles to stay in the pennant race. Its been a lot worse than we should have expected.
So is there a reason? I think its possible that in an effort to save the bullpen, starters have been left in the game too long. That has an immediate impact when they stop getting hitters out, but it also has a cumulative impact as the workload starts to wear them down. Ironically, the result is even more pressure on the bullpen making it in turn even less reliable.
Here is a comparison of the 2008 and 2009 season for the "young five", Baker, Liriano, Perkins, Slowey and Balckburn
2008 the five made 128 starts and threw over 100 pitches in 39 games.
2009 the five have made 93 starts and already have thrown over 100 pitches in 38 games.
Perkins is the only one of the "young five" whose highest pitch count this year has not exceeded last year's top count and his equaled last season.
In short, the "young five" have been asked to do much more this year. It is not unlikely, it was too much. And that may explain the current state of the rotation.