I did some looking into the building blocks of this defensive Bradley/Kiermaier thing.
On a per inning basis, Bradley gets more assists, makes more double plays and fewer errors than Kiermaier. But Kiermaier gets more chances. Now more chances does explain the errors difference, which is very slight. (JBJ has a substantial advantage in the other 2 categories.)
So how much is pitching staff? If your pitchers give up more fly balls, you get more chances. Or is it that Kiermaier simply covers more ground. I can only look at the first part right now. Factoring in FB%, K%, HR/FB% and IFFB%, the two staffs are very equal with regards to “playable” fly balls in the OF. Not enough to make any difference in the defensive metrics, though.(The teams differ by 3 FBs per opposing 10,000 ABs.)
So it appears Kiermaier simply makes more plays. That helps his case massively. But there are two other important factors that do slight JBJ.
1) Ballpark. Fenway has massive walls in left and straightaway center. A ball could hit these walls 12-15 feet up and still is considered “playable” in my numbers, when in truth, it is not. While UZR can account for these being unplayable and not penalize Bradley, it does change the hypothetical “lost opportunities.” The OF wall in Tampa is closer to 9’ high, giving a significant reduction in margin for unplayable “playable” flyballs in Tampa.
2) Betts. Betts covers more ground than Wyoming’s area code and takes plays from Bradley, and on the side with more outfield ground to cover and lower walls. Kiermaier has never played next to an elite defensive outfielder anywhere close to the caliber of Betts.
So is Kiermaier a better defender because he has better skills? Or because he has a better opportunity and Tampa needs him to be better?
If the Sox and Rays swapped center fielders, which team really benefits more? And which player does?