Bobby Jenks could be steal of the offseason

Bobby Jenks

JenksBobby Jenks is changing the color of his Sox, moving from the Chicago White Sox to the Boston Red Sox. What are the Red Sox getting? If you look beyond his inflated ERA, Jenks was the victim of bad luck in 2010.

Jenks was mostly the same pitcher in 2010 that he was from 2007 to 2009, and in some ways, he was better. Jenks generated more swings and misses and struck out a higher percentage of batters than he had in the three seasons prior while allowing fewer base hits.

So what was Jenks’ problem? Opposing batters were missing the White Sox gloves.

Bobby Jenks since 2007

Jenks allowed a .260 batting average last year, his highest since he joined the White Sox. This doesn’t seem to make sense considering the high rate of strikeouts, but opposing batters had a .368 batting average on balls in play against Jenks last year, almost 100 points higher than the .269 BABIP posted against him over the previous three seasons.

This was even more drastic when runners were in scoring position -- his BABIP with RISP was an astonishing 145 points higher last year than it was from 2007-09 -- where Jenks saw huge spikes in opponents’ batting average and slugging percentage, leading to more runs scored. If Jenks’ BABIP returns to his average over the previous three seasons and he continues to strike batters out at a high rate, the Red Sox could have the steal of the off-season.

Compared to this offseason's most coveted reliever -- former Tampa Bay Rays closer Rafael Soriano -- one appears to be an elite reliever while the other appears to be not very good at all.

Rafael Soriano vs Bobby Jenks
2010 Season

However, when we dig a little deeper, we find those characterizations might be very misleading. While Soriano's ERA (1.73) was substantially better than Jenks' (4.44), their underlying numbers actually support very similar ERAs.

Specifically, Fielding Independent Pitching -- which takes out the things for which a pitcher is not responsible, like defense -- says that the difference between Soriano and Jenks in 2010 was, largely, luck. Soriano benefited from a remarkably low BABIP, while Jenks' was one of the worst.

The net result, Wins Above Replacement, pegs these two relievers as virtually exactly as "valuable" as each other last season despite the wild difference in their ERAs.