Sean Murphy and Jed Lowrie also had RBIs for the A’s, who will try for a three-game sweep Thursday.
Burch Smith and Lou Trivino combined to pitch three scoreless innings for Oakland, and Jake Diekman completed a scoreless ninth for his fifth save.
Boston has lost three straight.
Kaprielian (1-0) settled down after a rocky first inning and ended his night after giving up just four hits and three walks with six strikeouts. The right-hander made his first start of the season at Triple-A Las Vegas last week but got the call after starters Jesus Luzardo and Mike Fiers both went on the injured list. He made two big league relief appearances last season.
He earned his first big league win in front of his father, an uncle and several friends who flew in to watch. He also did it thinking about his mother, Barbara Kaprielian, who died in 2014 after a 14-year battle with breast cancer.
The 27-year-old said he could hear his father’s voice every time the got to the top step of the dugout between innings. The victory was a long-time coming — as a minor leaguer, he missed nearly three full years due to Tommy John surgery and other injuries.
“They’ve been in my corner times when I didn’t think I’d be in this position,” Kaprielian said. “They are the backbone of who I am.”
Oakland pounded out seven hits and four runs against Boston starter Eduardo Rodriguez (5-1). He struck out nine but also balked in a run and didn’t get the run support he received in his previous six starts.
Boston entered the night leading the major leagues in runs (190), but it struggled with runners on base. The Red Sox loaded the bases with no outs in the first inning, had runners on the corners with no outs in the fifth, and runners at second and third with one out in the eighth. They’ve scored eight runs in their last four games after totaling 45 runs in their previous five.
“We know we’re a good offense. When the big boys are not swinging the bat, the other guys have to contribute, too,” Red Sox manager Alex Cora said.
Meanwhile, the A’s did just enough with their handful of opportunities.
With the score tied 1-1, Mitch Moreland led off the fifth with a single, followed by Elvis Andrus’ double. Rodriguez struck out Mark Canha, but Lowrie’s groundout to Xander Bogaerts was deep enough to score Moreland and move Andrus to third.
The lead grew to 3-1 when Rodriguez was called for a balk, scoring Andrus.
Kaprielian said he couldn’t have imagined his first major league start ending any better.
“My journey to get to this point was an absolute struggle. There’s time when it was great and times when it was bad,” he said. “At the same time, when my back is against the wall you’re never going to see any quit out of me.”
Athletics: Reinstated C Aramis Garcia from the injured list. He was on the opening day roster and hit .135 with a home run and two RBIs in 13 games before being placed on the 10-day IL May 4 with viral enteritis.
Red Sox: Placed right-handed pitcher Nick Pivetta on the COVID-19 injured list after he experienced side effects from his recent vaccination. Eduard Bazardo was recalled from Triple-A Worcester to take his roster spot.
Oakland center fielder Ramón Laureano was called out on strikes to end the A’s half of the third inning. He remained at the plate arguing as Boston’s players exited. He eventually threw both his batting helmet and bat and was ejected by home plate umpire Ryan Wills.
Laureano made a few steps toward Wills and pointed emphatically at Wills’ face before being restrained by members of the coaching staff. It was the second career ejection for Laureano.
Athletics: LHP Sean Manaea (3-1, 3.07 ERA) will make his eighth start of the season. He is 3-0 with a 2.23 ERA over his last six starts.
Red Sox: RHP Garrett Richards (2-2, 4.54) has gone at least seven innings in two of his last three starts.