The replacement of Mark Reynolds by Melvin Mora at third-base for the Arizona Diamondbacks, was a move of which a good number of fans disapproved. Reynolds was generally a fan favorite, famous for launching monster bombs at Chase Field, and so what if he struck out 200 times? An out's an out, right? Those who thought so were particularly unimpressed when new GM Kevin Towers signed the 39-year old Mora, with an OPS+ of 96 since 2005, to stand in Reynolds' stead. And even Mora's defenders will have had their confidence shaken by a performance yesterday which, by one metric, was among the worst on Opening Day in baseball history.
Win Probability Added (WPA) is a number which shows, directly, how your performance affected the team's chances of winning a game. Over the history of baseball, enough data has been accumulated to show that, say, if you're leading by three runs at the end of the sixth inning, your chances of winning are 81%. You can do the same for every situation in a game, and credit a player - pitcher and hitter - for the difference before and after each plate-appearance. Total that up over the course of a game, or even a season, and you can find out their overall contribution. Or plot it over the course of a game, and you get a nice picture of the ebb and flow.
For example, Mora's first at-bat yesterday was with one out in the top of the second, with the Diamondbacks' 2-0 down. Arizona's chances of winning when he came to the plate were 29.4%. After Mora grounded to the short-stop, they were 27.6% - he took away a little less than two percent. Here's how all his at-bats went:
- 2nd inning. One out, bases empty, 0-2 down. Ground-out to SS. -2%
- 4th inning. No outs, men on first and second, 1-3 down. Grounded into double-play. -13%
- 6th inning. One out, bases empty, 4-3 up. Reached on error. +3%
- 8th inning. One out, man on first, 6-6. Grounded into double-play. -11%
- 10th inning. One out, men on second and third, 6-6. Grounded into fielder's choice, runner out at home. -22%
Adding it all up, Mora's positive at-bats were worth +2.7% - his negatives -47.7%, for an overall total of -45%. That's an impressive sum. It's very hard for a position player to reach such a negative tally for a couple of reasons. Firstly, you only have a few plate appearances in a game. Secondly, a batter can never lose a game, they can only fail to win it. It's a lot easier for a relief pitcher (also with a limited number of chances to affect things) to get tagged with a huge negative WPA. For instance, Brandon Lyon in the Astros opener ended up at -90.7%, coming in with a 90.7% chance of a Houston win, and letting Philadelphia walk-off. Hitters can only do that on the winning side.
Eight players all last season in the NL reached -45% or worse, and Mora's performance - in his very first game for the Diamondbacks - ranked eighth worst all-time for Arizona. [What's even rarer is that his team still won. Mora is the tenth NL player since the end of 2004 to have had a WPA of -45% and still seen victory; most, like him, being in extra-inning contests, where a side can overcome your "contribution"] And it was the fifth worst number anywhere in the majors on Opening Day, since records began. #1, incidentally, belongs to Dusty Baker for the 1986 A's, who racked up -65.1%, mostly for hitting into a game-ending double-play.
WPA is far from a precise metric. There's no component for defense, for example - which is probably a good thing for Mora, as his misplay of an infield hit allowed the Rockies to score the tying run in the seventh. But If you hit a ninth-inning home-run, OPS doesn't care whether the score was tied or if it's a meaningless blow-out. WPA gives a player much more credit for the former, so provides a decent "fan's-eye" view of player contribution. If you were here in the thread yesterday, you'll have seen a lot of unhappiness about Mora's signing, small sample size be damned. It'd have been much worse if we'd lost.
And to some extent, that's justifiable. In the 563 games Reynolds played for Arizona, he never had a WPA worse than -33%, so for Mora to come in and demolish that in his debut will lead to a lot of "See? Told you so." On the other hand, Mora is not that bad, or at least, he's not going to be worth -45% every game. It was the worst figure of his career, and in over 1,500 games, just once before had his WPA contribution been lower than -31%. Only seven men in the NL have even had two -45% games in a season, so we trust Mora has simply got this stinker out of his system early.
What it does, is make a poor first impression on a fanbase who were already sitting on the fence regarding his presence on the roster. It's not a lethal blow: we are nothing if not a fickle bunch, whose favors can be won as easily as they are lost. But the confidence lost in Melvin yesterday will likely take more than one game of solid defense and timely hitting to rebuild.