Guerra was excellent in the role last season, posting a 3.30 FIP and converting 21 of 23 save opportunities. However, Guerra simply doesn't fit the profile of a consistently shut down closer, while Jansen and his mid- to upper-90s fastball does.
Therefore, while Guerra may break camp as the closer, Jansen should get the job at some point in the near future.
The prevailing wisdom is that it takes a special quality to close out games, and thus Guerra should get the job because of his success last year. However, an analysis of the top 15 relievers in terms of WAR (courtesy of fangraphs.com) shows that each and every one of them has a K/9 of at least 8.0.
This makes sense empirically as well: if we think about the most effective relievers of the last decade or so, a common characteristic is the ability to miss bats. To bring this back to the Dodgers, Jansen clearly has better swing-and-miss stuff, as evidenced by his 16.10 K/9 in 2011. Guerra’s was only 7.33.
Because an analysis of K/9 and WAR demonstrates that strikeout ability is a key factor in a relief pitcher’s effectiveness and it follows common baseball protocol to have a team’s most effective reliever pitching in the ninth inning, Jansen should be that man.
More than just strikeout ability points towards Jansen being the better pitcher, however.
One might expect that because Jansen has a higher strikeout rate, he would have less control. But Guerra is not exactly a control artist; in fact, Jansen’s WHIP last season was significantly lower than Guerra’s. Also, xFIP, which is calculated on the same scale as ERA, suggests that Jansen was nearly two full runs better than Guerra last season.
All of these numbers used to suggest that Jansen is a better pitcher than Guerra are last season’s big league numbers, and it actually wouldn’t be unreasonable to think that Jansen will only continue to get better. After all, he has only been pitching since 2009 while Guerra has been a pitcher since he got into the Dodgers system in 2006.
Guerra’s numbers throughout his minor league career have been consistently around what he posted last season, while Jansen’s control has improved each season.
Manager Don Mattingly has said that Guerra will open the season as the closer. However, numbers show that Jansen was a better pitcher last year.
If the Dodgers want to put the best arm in their bullpen in the closer role, then they would be well served to make the switch from Guerra to Jansen sooner rather than later.