After last night’s ugly loss, it is not all that surprising that the Marlins designated Nate Robertson for assignment (along with sending backup catcher Brett Hayes to the 15-day DL) and brought up reliever Jorge Sosa to replace him. This move comes with a few interesting implications, which I think are worth mentioning.
Was Nate Robertson as bad as advertised?
There is a good chance that he was. Though Robertson’s FIP was at a terribly mediocre 4.71, much better than his 5.47 ERA, there were definitely bad signs in his pitching all year. His skill-based ERA estimators, such as xFIP and SIERA, were not as kind; xFIP had Robertson’s performance as a 5.01 ERA and SIERA predicted a 4.95 ERA. Both systems expected a higher home run rate given Robertson’s 40.2% FB% (according to BIS/FanGraphs). Given that his career HR/FB% was around 12% and Robertson’s 2010 rate was at 8.3%, this adjustment is not surprising.
But it isn’t just that Robertson “should have” allowed more home runs than he actually did. For some reason, he has missed fewer bats this season than ever before. This season’s 13.6% K% is the lowest of his career, and the plate discipline and contact numbers FanGraphs provides reasoning for this problem. This season, hitters are making contact with 85% of pitches swung at, a career high. As a result, his 6.5% rate of swinging strikes (swinging strikes / total pitches) is also the lowest of his career. Robertson’s rates of putting the ball in and around the zone don’t deviate much from his career rates, suggesting that hitters are just seeing his pitches better.
Combine all of that with a projected BABIP that is higher than the average thanks to the last few seasons and you would expect this to lead to more hits and more problems for Robertson. At this point, he is not much more than a bullpen option for any team. He is definitely not much of a starter any longer.
Who takes his place?
There are two pitchers in line to replace Robertson in the Marlins’ rotation. At the moment, the team brought up Sosa for bullpen relief, as the Marlins will not need anyone to contribute in Robertson’s rotation spot for the next few days. Still, when that spot comes up, the Marlins will have to choose between recently demoted Chris Volstad and Sean West to replace him in the rotation.
I already posed my argument for Volstad last week; why demote a pitcher when he fixed (or regressed) the only thing he did really poorly last season? There is an argument that, given that Volstad was a former first-round draft pick, one would expect more from him than what he has done so far in his career. However, I would venture to say that he will never develop into a pitcher with overpowering stuff because he never had it in the first place. Volstad was not a guy with a blazing fastball or great breaking pitch who simply did not know how to control his arsenal. Rather, he always had the stuff of a mid-rotation guy who could keep the ball on the ground. He could still improve in this area, but he’ll never miss many more bats than he already does now.
West may yet develop more of a strikeout game, but at this point in his development, he is essentially a left-handed version of Volstad. There is also some concern that, since he had his season-ending injury in 2008, that he may not return to the form he showed in the low minors in terms of strikeouts and stuff. This season, he has pitched 43 innings in Triple-A New Orleans and had middling strikeout (19.6%) and walk (7.4%) rates. Translated to the majors, Baseball Prospectus had his Davenport Translation at a 4.93 ERA, not much worse than 4.78 ERA this season.
If I had to choose, I would go with Volstad, bypassing the use of at least one lefty in the rotation. I’ve already examined the minimal effect that would likely have on the pen, though having Robertson work out of the bullpen as a second lefty specialist could be of some use if the team went with a no-lefty rotation.
What may be even more interesting about this move is the opportunity that Alejandro Sanabia has now to hold a rotation spot. In 17 innings so far this season, he has been pretty impressive, striking out 17 hitters and walking four. He will get at least one more opportunity in the rotation after this upcoming series against the Colorado Rockies, so it will be his audition to remain in the rotation. West could arrive and replace him otherwise in order to fit in a lefty into the rotation if Sanabia falters. Right now though, the strikeouts look very good, even if the raw stuff itself is underwhelming.