With regular leadoff hitter Betts missing his second straight game because of tightness on his left side that he felt in batting practice Sunday, Benintendi added a triple and single, improving to 4 for 8 in the two days.
"I'm seeing it pretty well right now," Benintendi said in his usually quiet voice.
Reminded that it was his first career homer over the Green Monster, the 23-year-old said: "I've hit a few over there in BP, but not in a game."
Betts entered the day with a major-league-leading .359 batting average.
Jackie Bradley Jr. added an RBI double, scored twice and made a spectacular catch in center field as Boston won for the seventh time in nine games.
It was the Blue Jays' 11th loss in 15 games, dropping them to 2-5 against the Red Sox this season. Teoscar Hernandez had an RBI triple and double for Toronto.
David Price (5-4) labored through five innings, allowing two runs and four hits with four strikeouts and four walks.
The lefty took a hard grounder from Yangervis Solarte that bounced off his glove and appeared to carom off his chest in the first inning before picking it up and throwing him out.
"You know me, I'm the softest guy in this clubhouse," Price said, joking. "If that had bothered me, I was coming out of the game."
The Red Sox took charge with their big fourth inning, pulling ahead 6-1 against Aaron Sanchez (2-5).
Brock Holt had a sacrifice fly and Bradley his run-scoring double before Benintendi lofted a fly ball into the first row of Monster seats.
"They get a couple singles in that inning, a bloop, and then a blast," Sanchez said.
Martinez then led off the fifth with his 17th homer, a drive that completely left Fenway Park over the Monster, tying him with Betts for the team lead.
"They've got a tough lineup, they always have," Blue Jays manager John Gibbons said. "It's tough to shut `em down."
Sanchez was tagged for seven runs and nine hits in five innings, falling to 0-3 in his past five starts with a 5.96 ERA.
Blue Jays: 3B Josh Donaldson left the game with left calf tightness after getting a single and walking twice.
"It's a little sore right now, still a little tight," he said. "We're going to do some treatment and stuff on it and see, just kind of play it by ear, see how it responds."
Red Sox: Manager Alex Cora gave 2B Dustin Pedroia the day off after he played two straight games, his first since returning from offseason knee surgery.
LOOK AT THAT
Bradley made a jumping catch with his back completely turned to home plate on the warning track against Kendrys Morales in the sixth. Holt ran over, raised his arms and then walked over and gave him a hug.
"That's kind of what Jackie does," Benintendi said. "He makes those kinds of plays all the time."
Bradley said: "I always put a high expectation on myself offensively and defensively. I felt like I should make the play."
Toronto's Russell Martin made his first career start in left field. In his 13th season in the big leagues, the 35-year-old has been primarily a catcher. He was out playing caroms off the Monster before the game.
ALL RIGHTY THEN
The Blue Jays had three switch hitters and six batting right-handed, so that gave them an all-righty lineup against Price.
Blue Jays: RHP Marco Estrada (2-5, 5.40 ERA) is set to start Tuesday. He's 0-2 with a 7.36 ERA in two starts against the Red Sox this season.
Red Sox: Rick Porcello (6-2, 3.74) hopes to rebound from his worst start of the season when he gave up six runs over 3 2/3 innings against the Rays on Wednesday.
More AP baseball: https://apnews.com/tag/MLBbaseball
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