Slowly, Rob Johnson is gaining acceptance amongst our circles here. Why, I am not that interested in. Perhaps just enough time has passed for us to get used to the idea that he, Rob Johnson of all people, is our starting catcher that people have stopped caring. Another factor might be that with Jeff Clement gone to Indianapolis and Adam Moore and his .766 Tacoma OPS clearly not yet ready, there does not seem to be any other choice right? We know Kenji sucks. Right?
"Catcher's defense is not important" has become too much of a black and white mantra around here, another example of a nuanced and context-dependent assertion getting blown up into a mass media sound byte and applied all willy-nilly. Defense is important no matter the position. The deal with catchers is that
A) We cannot fully evaluate their defensive impact. We have two meaningful defensive metrics for catchers: how well they control the running game, and how well they block pitches (i.e. preventing wild pitches/passed balls). We have good solid understanding of these and how to translate these into runs.
B) Compared to other positions, the standard deviation amongst Major League catchers for the above mentioned metrics is just not that large. The best catchers at controlling the running game are usually around +5 runs per season, the worst -5. Ditto for blocking pitches. ~20 runs is the difference between the best and worst catchers.
Now, yes, if you paired up the worst in both with the best in both, you would get something around a two win difference. Two wins is big and meaningful, but that is also like Baseball Prospectus' old problem with replacement level players. To determine WARP, BP used to take a replacement level hitter (which they had a good measure of) and combine it with a replacement level fielder (which they had less of a good hold on) and use that to set replacement level for a player. That set the bar way, way too low because if you were at that level in both aspects, you were not hanging around Triple-A waiting for a job, you were in High-A if you were under 24 or else you were a coach.
In practice, the best and worst catchers in the Majors would be about ten runs apart in the defense that we could measure. Compared to other positions, that is tiny and thus defense is currently less measurably important of a factor at catcher than at another position on the field. Cleared up?
How does that apply to us now? League average caught stealing rates are stable at 26 to 27% each year recently. Kenji Johjima's rates have been 28%, 40%, 27% and 52% this year. That is remarkably good. Rob Johnson was bad in limited action last year, but is just under 30% so far this year, a fair number but still 10% behind Johjima's career rate. Even though that is a big difference in percentage, that still only translates to a handful of runs in Kenji's favor.
However, we have the other measurement that we have pretty well down; blocking pitches. Note that you should not just look at passed balls otherwise you let the scorekeeper into the mix. You want to consider both passed balls and wild pitches and, no surprise to those that watch Johnson futilely try to catch, this is where Johjima really shines compared to Rob Johnson.
Per nine innings caught, Kenji has rates of 0.38, 0.36, 0.34 and 0.34 passed balls + wild pitches. That might seem high, but the MLB average so far this year is just over 0.39 so Kenji comes in a little above average. Rob Johnson, on the other hand, yowza, has a career rate of 0.58, a whopping 0.22 more per nine innings than Kenji. Now, is that perfect? No, because we are not controlling for pitchers caught, but it is probably pretty close to washing out.
Thanks to Sean Forman, we have a figure to attach to each passed ball and wild pitch of 0.27 runs. For every nine innings caught to date, Rob Johnson has been about 0.06 runs worse than Johjima at the actual act of catching. Over the course of a typical 1,000-inning catcher season, that is 6.6 runs.
When you add up our two measures of catcher's defense, Rob Johnson falls about 8-10 runs behind Kenji, nearly a full win. That is a sizable gap to make up with in terms of hitting, and given their respective ZiPS' projections (.296 wOBA for Johnson, .294 for Johjima), that would equal just three-fourths of a single run over a typical catcher's season. Even their current wOBA*s (.286 for Kenji, .299 for Johnson) does not even the difference on defense.
It reads like I am trying to build a definite case for Kenji Johjima over Rob Johnson, but that is not really my aim here so let me include a couple important points. One, Rob Johnson is trending upward, his hitting is improving and he has age on his side. Kenji Johjima is clearly in decline, so while he seems like an obvious choice (by about 7-9 runs/1000 innings) at the moment, this is an always fluid analysis.
Secondly, this is based on the stuff that we can measure and it sure seems like Rob Johnson excels at everything that we cannot. The pitchers do love him, I will not deny that or deny that it has any effect at all, but I also think it has a massive hill to climb just to make them equal in value and I want people to understand what they are asserting when they claim that Rob Johnson is our best option at catcher.