The Miami Marlins took an aggressive step in trying to fix Derek Dietrich's defense this season by demoting him to Triple-A last night. Presumably, manager Mike Redmond and the rest of the Marlins are tired of having Dietrich's lead gloves allow for extra hits or runners for the opposing team, as those extras are costing Miami runs. Miami figures that, since they are still in something of a pennant race, they cannot afford to have those extra runs come through from Dietrich's defense.
But have the Fish forgotten that Dietrich, through this point in 2014, is actually a good hitter, having hit .246/.342/.431 (.345 wOBA) this season. Dietrich has quietly been the team's second-best hitter along with Garrett Jones behind Giancarlo Stanton. But the Marlins have conveniently forgotten this in favor of his defensive mishaps.
There must be a balance to having Dietrich's offensive and defensive contributions. Miami cannot simply have forgotten that Dietrich has hit well all season. Or perhaps the team does not know how to properly weigh his defensive mishaps versus his offensive prowess. Luckily for us fans, we can get an estimate of that by converting all of these performances into runs, which are a useful denominator for evaluating baseball. Just how many runs is Dietrich costing Miami on defense versus his replacements, Donovan Solano and Ed Lucas, and just how much is Miami losing out with Dietrich's in Triple-A?
The offensive part can be easily done. Dietrich may have been doing a very good job so far this season, but the season is only 150 plate appearances in and the projections are still mixed on his future performance. ZiPS is projecting a mediocre .237/.301/.404 (.312 wOBA) batting line going forward. It figures that Dietrich's plate discipline problems from the past should stick around to some degree, but the progress he made since last season in terms of power and batting average should stick around a bit as well.
What about his replacements? As of right now, both Lucas and Solano conveniently have similar projections. Neither hit the ball with much authority, and neither walk often enough to get on base much if their batting average luck falters. In total, you would expect a batting line akin to .245/.300/.320 and a wOBA around .280 going forward.
Needless to say, Dietrich's batting line is a good deal better than Lucas's and Solano's. Miami would be giving up some number of runs by turning to those two versus Dietrich. How many runs? The estimate is something like 0.025 runs per plate appearance, meaning that in any given game, Lucas and Solano are expected to be 0.1 runs worse than Dietrich. It's a 15-run gap in 600 plate appearances, or about a win and a half in difference.
Miami's argument for demotion is that Dietrich needs to work on his defense, but he could easily work on it in the majors. Their true argument is that, right now, Dietrich's defense is costing them more runs than his offense is providing, thus making him a lesser option. Could that possibly be?
If we look at his career defensive numbers available on FanGraphs, you can make an argument for the Marlins' case. Dietrich has been worth between nine and 13 runs below average in 798 2/3 innings at second base. If you take the average of 11 runs worse than average in that time frame, you get a rate of 0.124 runs lost per nine innings compared to the average second baseman. If the Marlins thought that Dietrich's career performance was going to continue, then he'd be costing Miami more runs compared to league average than Solano or Lucas, assuming the latter two are average defenders.
But that is not likely the case. Dietrich's true talent is more likely to be closer to average than his career defensive numbers so far. Beyond that, it is possible Lucas and Solano are a bit above average defensively. So how good does Dietrich need to be defensively to be worth it if he provides the projected amount of offense? For Dietrich to be better than Solano or Lucas, he would have to be worth almost nine runs below average in almost 800 innings going forward. That translates to a season rate of about 13 runs worse than average.
Even if you tack on a run or two more, Dietrich essentially has to be a Dan Uggla-level defender or a bit better than that going forward to justify his remaining in the lineup. Miami is banking on him essentially being a disaster on the field who needs drastic retooling. But if he is just a bit better than that, Miami should have a better overall player in Dietrich over the near-replacement-level fodder they plan on using. The Fish should feel safe knowing that they would likely not be giving up more runs opting for Dietrich, and it would still provide him time on the field to work out his obvious kinks at second base.