And it did.
The rookie, called up from Triple-A Reno on Aug. 7, connected with two out in the eighth inning. It was his second career homer.
''That was really cool,'' Lamb said. ''I missed a lot of fastballs early on in the game. It was more than frustrating. But I went up there, the guy was throwing hard, and I just wanted to have a short swing. When he's throwing that hard and you hit barrel on it, it can go far.''
Rex Brothers (4-6) walked his only three batters and threw a wild pitch before Ottavino came in. But Ottavino looked set to escape the threat before Lamb went deep.
He got Mark Trumbo to bounce into a fielder's choice that could have been a double play if the reliever was able to get the ball out of his glove cleanly. He also struck out Miguel Montero before Lamb came to the plate.
''Being conservative I just tried to throw a strike thinking I could overpower him,'' Ottavino said. ''He barreled it. Bottom line, he did a great job.''
Brad Ziegler (5-2) gave up a run in his one inning but got the victory. Addison Reed fanned two in a perfect ninth for his 31st save of the season and No. 100 for his career.
Reed said it was ''just a number.'' He was more interested in talking about Lamb's slam.
''If that would have been save number two I would have been happy,'' Reed said. ''Any time we get a go-ahead grand slam by a rookie that just came up, it's awesome. It has to be awesome for him too.''
Colorado grabbed a 2-1 lead on Charlie Blackmon's sacrifice fly in the eighth inning of the opener of a weekend series between the teams with the worst records in the National League.
Both starting pitchers were excellent.
Arizona's Josh Collmenter allowed one run and two hits in 6 1-3 innings. He retired 18 in a row after Blackmon led off the game with a single.
Collmenter's streak ended when Drew Stubbs reached on a bunt single down the third base line with one out in the seventh. Nolan Arenado then had a two-out RBI double off Evan Marshall.
Colorado's Christian Bergman, making his fifth major league start, didn't allow a hit until Collmenter doubled on an 0-2 pitch with two outs in the fifth.
In the sixth, David Peralta tripled to right-center. Bergman intentionally walked Trumbo, and then was replaced by Christian Friedrich, whose wild pitch allowed Arizona to score.
Rockies: RF Michael Cuddyer is on the DL for the third time this season, this time with a sore left hamstring.
Diamondbacks: Manager Kirk Gibson says LHP Daniel Hudson (two Tommy John surgeries), INF Chris Owings (left shoulder), OF A.J. Pollock (broken hand) and OF Cody Ross (calf sprain) are expected to join the club when rosters are expanded on Monday.
It will be Bob Brenly bobblehead night Saturday for the team's TV analyst and former Arizona manager. LHP Tyler Matzek (3-9) goes for Colorado, Vidal Nuno (0-3) for Arizona.
Bergman was making his second start after coming off the disabled list. In his first outing back, he allowed four runs in 6 1-3 innings and got his first major league victory as the Rockies beat Miami 7-4.
Five of Arizona's eight starting position players began the season in the minors.
A wild pitch by Brothers in his brief appearance bounced hard off catcher Michael McKenry's left shoulder. The trainer went out to check on McKenry but he stayed in the game.