See the previous article for a full explanation of what Win Probability (WP) is, how it's calculated, etc. Because Fangraphs don't have an easy search feature for WP, I'm using Baseball-Reference.com's version of the stat throughout this piece. It shouldn't make much difference, but I have noticed they aren't quite in line, perhaps because they use a different base period of history.
Position players
Best: Tony Campana, April 10 vs. Giants, +72.2%
While Campana's number wasn't quite good enough for the all-time franchise top 10 (it's #11), it was top this year by quite some way, with the runner up being a Miguel Montero game where he scored +55.5%. Campana was 4-for-6 in the game, though had only one RBI. That was the go-ahead run in the 10th, off Yusmeiro Petit, and was worth +37%. But for WP purposes, he was also credited with the tying run in the eighth inning, which came after he singled and the Obese Raccoon botched the throw over to first. That late momentum swing was worth +30%.
Worst: Miguel Montero, August 29 vs. Rockies, -36.3%
What's interesting is that the D-backs still actually won the game. The turning point for the entire game came in the bottom of the eighth inning: you may remember it, because it started with Rex Brothers walking the bases loaded without recording an out. He was replaced, and Mark Trumbo hit into a forceout at hoime for the first out. Miggy then came in and struck out, which whacked our chances of victory down by 20% [we'll get to what happened next, a bit later!]. That capped an 0-for-3 day for Montero: he also walked, but was then caught stealing on what was probably a blown hit and run.
Starting pitchers
Best: Vidal Nuno, July 8 vs. Marlins, +43.8%
The main factors which lead to a high positive WP for a starter are: not allowing any runs, not getting much run support, and working deep into games, when outs in close and tied situations tend to cause a bigger swing. Nuño didn't go particularly deep, working seven innings, but the game was scoreless until the middle of the fifth, before Arizona took the lead. It then stayed that way until he left after seven shutout innings. Unfortunately, he didn't get the decision, and the D-backs didn't get the win, as Addison Reed allowed two in the ninth to give the Marlins victory.
Worst: Wade Miley, April 28 vs. Rockies, -61.2%
The lowest WP results aren't necessarily the worst starts, because the latter tend not to last long enough to have an impact. It's more likely to be a back and forth slugfest: in the five worst starts by WP of 2014, the Diamondbacks scored an average of 5.4 runs. The very lowest saw Miley work six innings: he kept digging holes, but the offense kept tying things up, coming back from 0-1, 1-3 and 3-4 down, thereby replenishing the WP for Wade to pitch away again. He eventually left 7-4 down, and the Diamondbacks lost by a margin of 8-5.
Relief pitchers
Best: Eury de la Rosa, August 13 vs. Indians, +25.6%
Extended innings in close games are best for relievers, and this contest gave us the first- and third-highest bullpen WPs of 2014, back to back. This was the second game of a double-header in Cleveland, with de la Rosa coming in to relief Andrew Chafin after five shutout innings in his MLB debut. With the teams tied at zero, Eury then three scoreless innings, before handing the ball to Matt Stites, who worked the ninth and tenth, worth 25.4%. The Diamondbacks won 1-0 in 12 innings, with Randall Delgado getting the W, and Reed the save.
Worst: Will Harris, April 3 vs. Giants, -80.8%
You might expect Reed to own this category, with blown saves an obvious fount of negative WP, and indeed, he occupies spots #2-5 on the list. But Harris's performance trumps them all: He came in for the top of the eighth with Arizona up by two, and did get the first two outs without giving up the lead. However, an RBI single, intentional walk and three-run homer turned a 5-3 lead into a 5-8 deficit. It's still only good/bad enough for 13th place all-time, but was the second worst WPA by an Arizona reliever before the ninth inning (David Hernandez, -84.4%, 11/06/07).
Individual Play
Best: Jake Lamb, August 29 vs. Rockies, +63%
Worst: Martin Prado, April 10 vs. Giants, -30%
There's a certain symmetry here. Both the best and worst plays came in games the D-backs won. The best was in the contest which also gave us the worst single-game performance, Lamb's eighth inning grand-slam undoing the damage done by Montero earlier in the frame. Conversely, the worst single play came in Campana's start, the best single-game performance of the season. Naturally, Prado's highly negative impact was resulted from one of his many double-plays, hitting into a 1-2-3 twin killing with the bases loaded and no outs, when we were down by one run in the seventh.