All of us want to see our favorite teams maximize the value of every player on the roster. Generally, the most valuable pitchers on a team are in the starting rotation, as the sheer number of innings can offset a reliever's lower run rate. However, starting is really hard and most pitchers end up in the bullpen. What pitchers who have been in the rotation may actually be more valuable in the bullpen?
The two biggest factors in finding pitchers whose niche is in the bullpen are platoon splits and working through the order. An opposing manager can set up their lineup to combat a starter's strength against righties or lefties, but relievers can be used at optimal times for his advantage. This isn't ground-breaking, but a starter's platoon splits aren't as talked about as much as relievers.
A big platoon split does not necessarily mean he should be in the bullpen. If a pitcher is league average against his weak side as a starter, he will end up as a good starter overall. It's the pitcher that is below replacement level against their bad side that could end up as a more valuable reliever than starter. Tommy Hanson was a great example last year. He allowed a decent .245/.323/.396 line against righties, but his .295/.363/.526 line against lefties shows he very well could have been much more effective in the bullpen facing 65% righties instead of under 50% as a starter.
The normal repertoire of these types of pitchers are sinkers, sliders, and nothing else, as those two pitches generally create advantages against same-side hitters. If Justin Masterson doesn't rebound to an average level, he could return to be a quality back-end bullpen type, usually worth 1-1.5 WAR. Francisco Liriano is a lefty example of this type of guy, especially if his control doesn't straighten out. Amid his rough 2012 season, he still held lefties to a .221/.310/.293 line.
The ability to work through an order plays a big role in a starter's success. If a pitcher struggles his second and third time through the order, there will be a lot of 5 IP, 3 ER outings. However, there needs to be more than just struggles later in games; they have to be great the first time through. This phenomenon was shown by Tommy Hunter last year. As a starter, he breezed through the order the first go-round, opponents hitting .234/.272/.357. After that, hitters teed off to a .338/.371/.635 line. His doubling of K's and 5 MPH spike out of the bullpen shows how much the relief role can, but not necessarily will, help a pitcher.
This is not a campaign to get the aforementioned pitchers in the bullpen this season. Liriano got a multi-year contract in Pitsburgh, so they will give him every chance to succeed as a starter. Hanson is on the SP-thin Angels, so I can't imagine him moving to the bullpen anytime soon. Teams generally do a good job of noticing these advantages and moving such guys to the bullpen. If only they would start using their relievers in optimal ways...