How bad does Clement’s defense have to be?

The Mariners just signed Kenji Johjima to a three-year extension, angering a lot of Mariners fans given that the M’s have catching prospect Jeff Clement waiting in the wings. The one justification I’ve heard for the extension is that there have been a lot of concerns over Clement’s defense, and so a move to first base might be in order.

How bad would Clement’s defense have to be for him to have to move over to first base?

Well, in an article I wrote a couple years ago, I found that catchers were about eight runs a year worse at the plate than the average hitter, while first basemen were 13 runs better. In total, that’s a 21 run difference. In other words, if Clement is 21 runs worse as a catcher than he is as a first basemen, then the Mariners should consider moving him to first.

Is that realistic? Not really. According to the projections we did for The Hardball Times Season Preview, the worst defensive catcher in baseball—Josh Bard—was projected to be -9.5 runs per 130 games in 2008. Let’s say that Clement takes over that mantle, and is -10 runs per season. Then, for it to make sense to move him over to first base, he would have to be a +11 fielder at first. Per 150 games (a full season for a first baseman), only four first basemen project to be that good in ’08: Doug Mientkiewicz, Albert Pujols, Lance Niekro, and Casey Kotchman.

So essentially, for it to make sense to move Clement over to first base, he would both have to be the worst defensive catcher in baseball and a top-five first baseman. That seems like a pretty unlikely combination.