The victory clinched the D-backs' third straight series win at home after they had lost six straight. With a win Sunday, the club will move to .500 for the first time since May 5, when it was 14-14.
"We were scuffling five games ago and we did a lot of talking on how we needed to reverse it," D-backs manager Kirk Gibson said. "We committed to ourselves. We needed to get rid of all the negative things around us. We've played good recently."
Trevor Cahill picked up the win against his former team, allowing two runs on seven hits while striking out seven in 7 1/3 innings. Cahill walked three, including Coco Crisp and Seth Smith in the eighth inning before Gibson summoned Brad Ziegler from the bullpen.
"Walks get him in trouble sometimes," Gibson said. "He was out of gas at the end. We were trying to get as much out of him, but he threw a great game for us. He was pretty much in control."
Ziegler, another former A's pitcher, allowed a single to Brandon Inge to load the bases before Jonny Gomes' sac fly scored Crisp, but the righty got Kurt Suzuki to ground into a forceout to end the inning and close the book on Cahill.
"Trevor was great. He made good pitches, he worked fast, got quick outs and you can't ask much more," Montero said. "It's tough, he probably wanted to do too much early, but he calmed down and had a really good game."
Cahill admitted facing the club that drafted him out of high school for the first time conjured up some extra feelings.
"I was probably more nervous than any other start this year," he said. "Going against the team I played for for six years. It definitely was a little more nerve-wracking."
Paul Goldschmidt's career-best hitting streak was in danger, as the first baseman was 0-for-3 with three strikeouts heading into the bottom of the eighth inning, but he extended his stretch to 16 games with an RBI single as the D-backs added a pair of insurance runs.
"I guess it's cool," Goldschmidt said. "I haven't really thought about it, I'm just trying help us win. Any time we can add runs, it's good."
The D-backs also extended their franchise record streaks of games and innings without an error to 12 and 111.
A's starter Jarrod Parker, who was sent to Oakland in the trade that brought Cahill to Arizona, performed well in his return to Chase Field through the first four innings, but his night unraveled in the fifth to the tune of five D-backs runs. He left after five innings having matched a career-high by allowing six earned runs on eight hits.
"Well, it came down to the Montero at-bat," said A's manager Bob Melvin, who was ejected at the end of the fifth for arguing balls and strikes. "I thought [Parker's] stuff was good, probably a little bit more behind than we've seen him. He just got a little frustrated and then, really, that pitch to Montero over the plate."
Tied at 1 to start the fifth after the D-backs left a runner in scoring position in both the second and third innings, Justin Upton broke the tie with an RBI double before Montero's grand slam later in the frame.
"He's swinging it better. He's worked hard," Gibson said of Upton. "He's more relaxed. He's got good energy and doing what he's supposed to do."