How Do Days Off Between Starts Affect Performance?

Ron Gardenhire, like most managers, usually keeps his starting rotation in a continuous order and doesn’t stray from that order, including maintaining the same rotation after off-days (barring injuries or poor performance). However, is there an opportunity for Gardenhire to better utilize each starter, based upon the amount of rest in between starts? Is there an optimal amount of rest for each starter?

As a group the Twins’ starters pitched slightly better when going on four days rest in 2010. Their cumulative ERA, WHIP, and HR/9 are better on 4DR, with the strikeout and walk ratios remaining the same as 5DR. However, pitcher-by-pitcher there’s quite a large difference in performance with four or five days off between starts.

The data below shows that, of the six pitchers who started the most for the Twins in 2010, three starters were better on 4DR and three were better on 5DR. For some of the Twins, the difference in individual performance was significant with an extra day off.

The best example is Scott Baker. He was an elite pitcher in 2010 with 4DR, posting a 2.88 ERA, 1.06 WHIP, and a 4.33 K/BB ratio. Contrast that with his stats on 5DR, where Baker was a completely different pitcher. His ERA and WHIP jumped to 5.82 and 1.51, respectively, with a 4.0 k/BB ratio. However, it looks like this difference may be more of an aberration than something that can be acted on in 2011. First, Baker was somewhat unlucky on 5DR as batters had an extremely high BABIP of .595 compared to just .370 on 4DR.  The .595 BABIP in 5DR stats should come down, so Baker’s ERA and WHIP in 5DR stats will improve. Second, Baker’s career stats are much closer to each other, with a 4.00 ERA, 1.22 WHIP, and 4.0 K/BB on 4DR and a 4.30 ERA, 1.29 WHIP and 2.9 K/BB on 5DR.   Thus, it looks like 2010 was an aberration and Baker’s 2011 numbers should move closer to his career averages.

Two other interesting cases are Francisco Liriano and Kevin Slowey. The above chart shows that both Liriano and Slowey were noticeably better on 5DR in 2010. However, unlike Baker, this split also affects their career numbers. Liriano has a 3.61 ERA and 1.23 WHIP on 5DR versus a 4.44 ERA and 1.34 WHIP on 4DR for his career. Slowey’s career split is even larger, posting a 3.47 ERA, 1.11 WHIP, and 6.61 K/BB with 5DR against a  4.89 ERA, 1.39 WHIP, and 4.41 K/BB.

There may not be any advantage to pitching Baker on a certain number of days rest, despite what the 2010 data shows, but it appears that there’s an opportunity to maximize the results from Liriano and Slowey by pitching them more often on 5 days’ rest. And with the Twins having six contenders for the rotation, could it make sense to have Blackburn or Duensing (whomever doesn’t make the rotation) as a spot starters in order to line-up the days of rest properly? That’s a topic for future post…

Tags: Francisco Liriano, Kevin Slowey, Minnesota Twins Blog, Scott Baker