There seems to be some generalized angst about the bullpen as of late. Losing a string of one run games, some in extra innings, will do that to you, I suppose. The Fernando Salas for Chuckie Fick swap seems to contain elements of that angst given that Salas continued to have strong strikeout rates, though he'd been struggling with control.
In the month of May, the Cardinals have lost 14 games. Of those, 8 have been lost by either 1 run or 2 runs. Generally speaking, these types of 1 run games even out over time. With considerations for home field advantage and a team's true talent, there's nothing particularly predictive or telling about ball clubs that lose games by just a couple runs.
That doesn't mean the losses aren't happening, however. Nor does it exonerate a bullpen that does appear to be part of the problem. Win probability added (WPA) is a measure of how much an individual event moves a team closer towards winning a game. In a context neutral environment, each team should have about a 50% chance to win a game. If a player hits a home run and puts their team ahead in the 1st inning, that event has increased their teams probability of winning game by some incremental amount. By summing all the incremental amounts from individual events over the course of a game, we can get a picture of how much each player contributed to the win (or the loss).
There are issues with WPA -- a homerun in the first is not necessarily as valuable in WPA as a homerun in the 9th or even as a single in the 9th -- but it gives a semblance of a picture about performance. (Remember that since each team starts the game with a 50% chance to win, .500 of WPA is equivalent to a player contributing 1 win on paper to the team.) Take a look at the following.
Setting aside the 7 innings that Eduardo Sanchez, Brandon Dickson and Chuckie Fick have pitched out of the pen, if it feels like the pen hasn't been pitching well, it is because they have been getting very poor results. WPA is a better descriptive tool than it is a predictive one, which is why you see peripherals and FIP listed as well. Fernando Salas and Jason Motte have each cost the team almost 2 wins based on their results in games. When you pitch late innings in close games, individual events are capable of wild swings in win probability. So Motte and Salas are both subject to accumulating significant amounts of WPA in their outings. Regardless, they've seen more bad events happen while on the mound than good ones to date.
WPA doesn't make any attempt to separate luck from those events though. So when Jason Motte allows a string of base hits that find holes in the defense and results in a loss, WPA punishes him heavily. Even if he was pitching well.
Conversely, Kyle McClellan and J.C. Romero were both being marginalized and used in games where the outcome was already mostly decided (read: mop-up duty in blow out games). That reduced their ability to impact games and, based on their peripherals, reduced their accumulation of negative WPA. Their lines could appear much worse.
The take away here is simple: Yes, the bullpen has been losing games. No, that doesn't mean the bullpen is bad.
Both Jason Motte and Mitchell Boggs have been performing well out of the pen -- even if Motte isn't quite the shutdown reliever he was last year. Fernando Salas was more of a mixed bag given the walk rate but clearly something was working as he struck out better than a batter an inning. After those two, Rzepczynski and Marte are both performing reasonably well and their peripherals would indicate that there isn't much to worry about (not that we should be doing too much tea reading off of <20 innings of data).
It's hard to say whether Eduardo Sanchez will be able to translate some of the success he's had in the minors to the majors. There's every indication that his stuff will play up but he's been hampered by some shoulder issues this year and his command has disappeared at times leading to wild spurts. If there's a player that can salve the discomfort right now, Sanchez's electric stuff could do so with a few lights out performances.
On the whole, however, there just isn't a lot of evidence in the statistics that these bullpen losses late in games are going to continue. Reliever performance is notoriously volatile but between Jason Motte, Mitchell Boggs, Eduardo Sanchez and Fernando Salas, the Cardinals should be able to piece together a successful core to handle the close and late innings that games can pivot on.