Collmenter was roughed up by the Padres on Sunday to the tune of six runs, seven hits and three walks in four innings, as the rookie right-hander dropped to 9-9 on the year, while watching his earned run average rise to 3.40.
"That guy has been throwing the ball well all year," Padres manager Bud Black said of Collmenter. "He's a strike-thrower and he threw some good pitches that we laid off and took some walks."
Collmenter has faced the Padres three times (two starts) this season and is 1-1 with a 4.91 ERA.
San Diego will counter with lefty Wade LeBlanc, who is 3-5 with a 5.43 ERA. LeBlanc snapped a three-start losing streak on Sunday by beating Collmenter and the D-Backs, as he allowed four runs and four hits in 5 2/3 innings.
"The first five innings, that's what I'm looking for ... throwing strikes, changing speeds," said LeBlanc, who added two hits at the plate. "Those are the things that I need to do to be successful."
LeBlanc is 2-2 lifetime versus the Diamondbacks with a 5.86 ERA in seven starts.
San Diego drew first blood in this series on Friday, as Tim Stauffer worked 6 2/3 scoreless innings to lead the Padres to a 2-0 win. Jason Bartlett collected two hits and scored a run, while Jesus Guzman picked up an RBI single for San Diego, which snapped a three-game losing streak.
Stauffer (9-12) scattered four hits and struck out six batters without issuing a walk.
"I felt good early on. I had a little extra on my arm. For some reason it was coming out better than on other days," Stauffer said.
Heath Bell pitched a perfect ninth to earn his 38th save of the season
Chris Young came up with two hits, including a double, and Willie Bloomquist had a double for the Diamondbacks, who have lost two straight and saw their lead atop the National League West standings cut to six over the San Francisco Giants.
Wade Miley (3-2) went six innings in defeat, giving up two runs on five hits while fanning three and walking three.
Arizona has won 10 of its 16 meetings with the Padres this season.