Turns out he was wrong.
Eight of 20 home runs off Tazawa in his big league career have been hit by Blue Jays batters.
"I looked at the video, and the pitch to Valencia leaked back quite a bit and went inside," Tazawa said through a translator. "I missed my location."
Tazawa said his coaches suspected he'd been tipping pitches against Toronto, so they made some changes.
"I'm using my breaking balls a little bit more, slide stepping," he said. "I think I've been doing a better job. Today, obviously, I didn't pitch as well, but I think I'm making some improvements."
Catcher David Ross said he doesn't understand why Tazawa, who came in with a career 6.52 ERA against Toronto, has fared so poorly against the Blue Jays.
"If I had an explanation, we'd find an answer," Ross said.
Jose Bautista homered for the second straight night, a two-run drive in the first that helped the Blue Jays improve to 7-16 in August.
Dioner Navarro's at-bat was prolonged when Ross dropped his foul popup for an error.
"With the wind swirling, it pushed back on him," Red Sox manager John Farrell said.
After Navarro singled, Tazawa came in, and Valencia turned on a 2-0 pitch for his first home run since Toronto acquired him from Kansas City on July 28.
"Obviously we've been struggling, every win is important," Valencia said.
Kevin Pillar followed with a double, was sacrificed to third and made it 5-1 when he scored ahead of shortstop Brock Holt's throw on Jose Reyes' fielder's-choice grounder.
Marcus Stroman (8-5) allowed two runs — one earned — and five hits in 7 2-3 innings. He had been 0-3 in four starts since winning at Boston on July 29.
"He was terrific today," manager John Gibbons said. "That's what the kid is capable of."
Ortiz, who returned to the lineup after sitting out the previous two games with a sore right foot, gave the Red Sox their first hit with a one-out single in the fourth.
Ross reached on a fielding error by third baseman Juan Francisco in the sixth, and back-to-back singles by Holt and Pedroia loaded the bases for Ortiz, who lined an RBI single to right. One pitch later, Holt scored on a wild pitch, but Stroman retired Mike Napoli on a groundout and struck out Daniel Nava.
Boston recalled LHP Edwin Escobar from Triple-A Pawtucket and optioned RHP Heath Hembree to Triple-A. Acquired from San Francisco in the deal that sent Jake Peavy to the Giants, Escobar was 0-2 with a 4.28 ERA in five starts at Pawtucket. Escobar made his major league debut in the eighth, retiring the side in order.
Out in the upper deck in left field, around the spot where Napoli's 451-foot homer landed Tuesday night, a Red Sox fan hung a target with Boston's red 'B' in the center. Next to it was a sign that read 'CAN U DO IT AGAIN NAP?'
LIKE FATHER, LIKE SON
Farrell's son Luke, a right-handed pitcher in the Kansas City Royals system, made his Double-A debut with Northwest Arkansas on Wednesday night. He took the loss, allowing six runs and eight hits in 5 2-3 innings against Tulsa.
Red Sox: SS Xander Bogaerts has passed baseball's mandatory concussion tests and been cleared to rejoin the team in Tampa Bay on Saturday. Bogaerts sustained a concussion when he was hit on the helmet by a pitch from Seattle's Felix Hernandez last Friday.
Red Sox: Boston is off Thursday before a four-game series at Tampa Bay starting Friday. The Red Sox have not announced a starter to face Rays RHP Chris Archer (8-6), who is 0-1 with a 4.32 ERA in three starts against Boston this season.