Remember when the Red Sox picked up Eric Gagne? It was a move guaranteed to make an already solid bullpen into the class of the league. Do you also remember how he then proceeded to allow ten earned runs in the next seven games, ultimately finishing his time on the Sox with a 6.25 ERA? The Red Sox sure do, and if Sean McAdams is to be believed, it's part of the reason the Sox are so wary of making a deal for a top reliever this trade deadline.
Sox are wary of having closers set-up thanks to Gagne experience.
But it's not just about closers and pressure scenarios. It's about all relievers. Before the season, Theo Epstein was reluctant to guarantee money to pen arms, instead signing a large number of minor league deals hoping to find a diamond in the rough (Scott Atchison is as close as he could come). He is not alone in his lack of trust for relievers. Just ask the best one on the team, Daniel Bard:
A guy could have success for another team and could struggle for us. It's happened in the past.
Bard wasn't even here for Gagne, and he's clearly suffering from PGSD all the same. Billy Wagner last year was an exception, as it was a very low risk deal with the high probability of at least draft pick compensation. Ask any executive on the team to give something serious up for an impact arm, though, and I expect they'll start shaking as if they just heard the crack of a bat.
But if all relievers suffer from inconsistency and unreliability-if only due to small sample size-that's just a reality all teams have to deal with. The only thing to do is minimize the unreliability and inconsistency by picking up the best guy available. It doesn't mean the Sox should overpay for what they're getting, but they also shouldn't clam up just because they could end up getting the bad parts of a good player's season. All deals carry risk, but not nearly so much risk as going into battle without a bullpen.