Trevor Rosenthal as a closer and potential starter

From 1992 through 1995, Mariano Rivera started 51 games for six different New York Yankees minor league affiliates. Even when Rivera was promoted to the majors during the 1995 season, he started in ten of his nineteen MLB appearances. However, after mediocre production as a starter, Rivera was moved to the bullpen in 1996, where he spent the next 1,096 appearances of his career forging a legacy as the greatest closer in the history of baseball.

The Rivera path is far from an uncommon one: a young pitcher begins his professional baseball career as a starter, it doesn't quite click like anyone wishes it would, and upon reducing his workload with a role in the bullpen, he becomes a potent weapon in relief. Trevor Hoffman, second to Rivera in saves in MLB history, began his minor league career as a shortstop and third baseman, but even with his lack of pitching experience, Hoffman started 11 games in the Cincinnati Reds organization in 1992.

The reason that big-league clubs would prefer their top pitchers become starters rather than relievers is intuitive enough: starters pitch more, and a team would rather its best pitcher throw 200 innings in a season than 50 innings. But things do not always work out as simply as putting the best guy in the rotation and put the next-best guys in the bullpen.

Trevor Rosenthal debuted with the Cardinals in July 2012 as a reliever. In 2011 and 2012, Rosenthal had started 42 games between AA Springfield and AAA Memphis, while pitching in relief in zero games. His promotion to the Major League bullpen was a reflection not only of his skills but of the team's need. The rotation was already solid: it had Adam Wainwright, a career year from Kyle Lohse, 2012 All-Star Lance Lynn, and the steady Jake Westbrook. Meanwhile, when the bullpen had an opportunity to add a flamethrower who averaged 97.4 miles per hour on his fastball, it's hard to pass that up.

In 2013, with the somewhat more ballyhooed prospect Shelby Miller taking Kyle Lohse's rotation spot, Rosenthal was back in the bullpen, having a dynamic season which culminated in his ascension to the de facto closer role in September after Edward Mujica fell out of favor in the ninth inning.

From 2013 through 2015, during his first three full seasons in St. Louis, Trevor Rosenthal ranked 8th among MLB relievers in Fangraphs WAR. From 2014 through 2015, in his two seasons as full-time closer, Rosenthal has led MLB in saves.

Criticizing relief pitchers is a little like criticizing politicians: no matter your specific allegiances, the generally safe play is negativity. Looks like those clowns in the Cardinals bullpen did it again. What a bunch of clowns.

But in Rosenthal, the Cardinals have a standout. Here are the five best seasons from a Cardinals reliever since 2000.

In Jason Isringhausen, the Cardinals had a strong option at closer, but he came at a price, having arrived in 2002 on a four-year, $27 million free agent contract. Pat Neshek was a diamond in the rough, arriving on a minor league contract, and as such, he was destined to only be a one-year wonder for the Cardinals. A terrific one, a worthwhile one, but he was a player who would need a big payday, which he instead received from the Houston Astros, to remain in St. Louis.

By contrast, Trevor Rosenthal has been making the league minimum up to this point in his career. And although he has now reached salary arbitration and his salary has jumped to $5.6 million in 2016, he will still make far below his market value.

His market value as a near-elite closer, no less. The market for starters is a whole different level: Ian Kennedy, a below-average starter in two of the last three seasons, earned 2.5 times Rosenthal's 2016 salary per year for the next five years on the free agent market, and the Royals forfeited a first-round pick for the privilege of doing business with him.

Regardless of his role, the Cardinals have three more cost-controlled years of Trevor Rosenthal ahead of them. If Rosenthal could (and this is unlikely, based on the general trajectory of pitchers) duplicate his relief performance in a starting role, his salary would certainly increase for 2017 and 2018, but he would still be an unmitigated bargain.

The general rule of thumb, however, is that when a pitcher moves from the bullpen to the rotation, he adds about a run to his earned run average. In 2015, Rosenthal had a 2.10 ERA, which means the starter version theoretically would have a 3.10 ERA.

While this doesn't make him Clayton Kershaw or Jake Arrieta, this would put him in the company of Scott Kazmir. While Scott Kazmir may not be an ace, he's the kind of pitcher any team would want in its rotation. He's the kind of pitcher who can command $48 million over three years as a free agent, as evidenced by the Los Angeles Dodgers this offseason.

But this evaluation of Rosey is a bit, pun intended/obvious/regrettable, rosy. It makes three fairly large assumptions.

1. That Trevor Rosenthal would pitch at exactly a one run worse rate as a starter. While with the Dodgers, Eric Gagne had a 4.68 ERA and 4.97 FIP in 265 1/3 innings as a starter and a 1.93 ERA and 1.90 FIP in 280 innings as a reliever. For a variety of reasons, some guys translate to certain roles better than others.
2. That 2.10, Rosenthal's career low ERA, is his true talent. At his 2016 ZiPS projection with an additional run tacked on, he has a slightly lower ERA than Edinson Volquez, which is acceptable though less alluring. If he reverts back to his 2014 self, his starter equivalent ERA of 4.20 would rank 63rd of 79 qualified starters in 2015.
3. That it's worth the trouble in any way, shape, or form.

To expand on #3: the current Cardinals starter projected by ZiPS to have the worst 2016 by ERA is Mike Leake, at 3.77. The second-worst is Michael Wacha, at 3.51.  The Cardinals rotation is certainly not lacking in depth on the back end, and while Rosenthal may very well have the capacity to be an above-average starting pitcher, it is not as though he would be replacing a replacement level pitcher. He would be replacing Jaime Garcia, Mike Leake, or even Carlos Martinez, none of whom are worse than average.

There was once a time when Trevor Rosenthal being moved to the rotation made all the sense in the world. The rotation was aging (Westbrook) and leaving (Lohse) and it's easy to forget today that half a decade ago, Kyle McClellan was the next man up in the Cardinals rotation.. And today, the options are so strong that the Cardinals can afford to have a dominant closer while another team may be forced to move a pitcher in Rosenthal's mold into a spot in the middle or back of its rotation.