Are Defensive Replacements Net Beneficial?

I was reading Rob Neyer’s latest column about the lack of offense from Braves reserves DeWayne Wise and Mike Hessman when I began to wonder why Schuerholz and Cox would put them on the team. Neyer makes a convincing case that these guys don’t have big league offense, but what about their defense? Well, Wise seems solid — at least as far as our sorry defensive metrics can tell us. Though his career has been short he has never committed an error. In addition his range factor is above the league average for outfielders. (Hessman’s MLB career has been too short to learn anything about his fielding from the stats, so I won’t focus on him.) Wise may be a lefty, but he is only a .490 OPS lefty who has probably only platooned in his carrier, so knock him down to a .400 OPS everyday player. He had a good spring (.318 BA), but this is not comparable to MLB numbers due to the weaker competition. Therefore, I think it is really pushing it to say he made the team for any reason other than good defense in the outfield. When the Braves have a one-run lead going into the ninth I expect to see Wise replacing Chipper in left…but is this a good idea?

This leads me to the more interesting question, are defensive replacements good baseball strategy? The general baseball wisdom is that when you have a small lead you will sacrifice offense for defense to protect your lead. But I am not convinced that this is a good idea. Let’s assume that Wise is a better fielder than Chipper (although their ZR’s are very close). Bobby Cox has to maximize the net difference in runs between the Braves and his opponent. If he leaves Chipper in game there is a slight chance that the other team will hit a ball that Chipper will miss but Wise could get. Of all the possible places a batter could hit the ball, the chance that it lands in a “marginal zone,” where Wise can convert a ball that Chipper could not into an out, is very low. I suspect the chance that the Braves lose the lead is not much affected by Wise being in the game. But, if Chipper is still in the game after the lead is gone his bat is available to produce runs. Putting Wise in the game decreases the likelihood that a ball in left-field will fall for a hit, but if more runs are needed (as they might be in a close game) you might be better off pinch-hitting Russ Ortiz.

The point is, I don’t think this tradeoff is a good one. My hunch is that there is a greater chance of needing Chipper’s contribution on offense than the chance of needing Wise’s small improvement on defense. And in a one-run game, Chipper’s offense is very valuable, so I would not take him out for a defensive replacement. Also, this might vary by position. While I think most outfield defensive replacements are not worth the trade-off I would guess that infield replacements ought to be more valuable. If anyone out there knows of any studies on this issue, I would be interested to know about them. I have found nothing on this in my web searches. I am toying with a method to test this, but I think it will be difficult.

Update: Just after I posted this I saw that Mac at Braves Journal has a good post on this topic.

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