The teams head into the season finale on Sunday afternoon. All that's left for the Rockies is an opportunity to prevent the D-backs from finishing above .500. The Rockies had a 4-3 lead against Webb, who gave up seven hits and struck out seven in seven innings, before Corpas (3-4) struggled.
Adam Dunn, who homered in the fourth off Rockies starter Glendon Rusch, led off with a double, and Mark Reynolds tied the score with a single. Corpas hit David Eckstein and gave up an infield hit to Jeff Salazar before Drew's go-ahead single.
Chad Qualls pitched the ninth for his ninth save.
"His last three outings he hasn't been able to establish his arm slot, getting depth to his pitches," Rockies manager Clint Hurdle said of Corpas. "His arm seems to be dropping a bit. He's not where he needs to be.
"That being said, I am not going to make excuses for him. But there's a whole different dynamic this year from the start to the finish, 75 outings. There might be some fatigue. We have to get him through this. He didn't handle the left-handers well, and that has been the problem the last three times out."
Last year, Corpas threw in 78 regular-season games, then went 1-0 with five saves in six appearances in the postseason as the Rockies made it to the World Series. The club signed him to a four-year $8.025 million deal through which he can earn more money and years.
However, by the end of April, Corpas had faltered to the point Hurdle put Brian Fuentes, a three-time All-Star, back in the closer role. For much of the year, Corpas has been good more often than bad, but the bad habits remain a concern.
Hurdle said Corpas' side work with pitching coach Bob Apodaca has been remarkable. His sinker and slider have downward break. Because they go in opposite directions, they can give hitters fits. But when the pitches don't sink, they're easy to hit.
If the Rockies are outbid by higher-paying closer-starved clubs for the services of Fuentes, who is headed into free agency, they're going to have to be confident Corpas can keep slumps short.
"That's something he has to handle himself," Hurdle said. "He's been around long enough to read the action on his pitches and get to the point where he can make that adjustment. The velocity is still there, so the fatigue thing I'm kind of moving to the side."
Webb, who gave up four runs and six hits in seven innings and finished the year with a 22-7 record, hit Chris Iannetta and Clint Barmes to begin a rally. Adam Melhuse knocked an RBI double, and Seth Smith and Ryan Spilborghs had RBI singles.
But the D-backs' Brandon Lyon (3-5), who threw a spotless eighth, wound up the winning pitcher.