Millwood, who allowed two runs and two hits, left after five innings with stiffness in his shoulder.
"It's not the way you want to be when you're out there," the veteran right-hander said. "I felt like, for the most part, I could still throw the ball where I wanted to, there was just really nothing on it."
Olivo had no idea his batterymate was hurting until the final out of the fifth.
"He looked fine to me," Olivo said. "I hope he's going to be OK for his next start."
Mariners manager Eric Wedge, who praised Millwood for being "a warrior," said it was too soon to say whether Millwood might need to skip a start.
"Too early to really talk about it," Wedge said. "We'll see how he feels when he comes in here the next couple of days."
Millwood set down the first 10 Toronto batters and retired his final five, but Wedge wasn't about to push him any further once he'd done enough to earn the win.
"He gritted through that last inning but I just didn't feel right pushing him back out there," the manager said.
How did Millwood explain his success in the face of pain and stiffness?
"I threw it pretty much where I wanted to, the ball was moving pretty good," he said. "I guess they weren't expecting 85, 86 (mph)."
Millwood (6-12), who entered 1-6 in nine career starts at Toronto, won back-to-back starts for the first time since winning three straight from May 13-23. The right-hander, who walked one and struck out three, has won just three of 19 starts since.
Edwin Encarnacion touched up Millwood for his 39th homer, a two-run drive into the second deck, but it wasn't enough to keep Ricky Romero from losing his 13th straight decision. The Blue Jays managed only two hits against five Mariners pitchers, with Seattle's bullpen working four hitless innings.
"Those aren't easy save opportunities we're seeing him get," Wedge said. "He's done a great job for us."
Romero (8-14) allowed three runs and eight hits in four-plus innings. Pitching on nine days' rest after giving up seven runs in one-plus inning against Tampa Bay on Sept. 2, the left-hander was unable to make the most of the extra down time. He remained winless since June 22 at Miami.
Romero struck out three and walked four, boosting his AL-leading total to 94, and matched Tom Underwood's team record of 13 straight defeats set in 1978 and 1979.
"It's frustrating," he said. "I'm sure everyone in this clubhouse wants to see me get out of it."
Olivo got the Mariners on the board with a drive to left in the second, his 10th. Mike Carp and Trayvon Robinson followed with singles, but Romero struck out Brendan Ryan and got Dustin Ackley to fly out.
Seattle put runners at second and third with two outs in the third, but center fielder Colby Rasmus caught Olivo's sinking liner to end the threat.
Romero wasn't so fortunate in the fourth. Ryan drew a two-out walk, moved to third on Ackley's double and both runners scored when Franklin Gutierrez singled to right, with Ackley evading catcher J.P. Arencibia's tag at the plate.
Pitching coach Bruce Walton came to the mound after Romero issued a four-pitch walk to Jesus Montero to begin the fifth. When Michael Saunders followed with a ground single to right, manager John Farrell came out and replaced Romero with former Mariners reliever Steve Delabar. Romero did not make eye contact with his manager as he handed off the ball and walked back to the dugout.
"They had me against the ropes a few times and I was able to get out of those jams," Romero said. "It didn't play out like I wanted it to."
Farrell insisted Romero will start again when his turn comes up next, Tuesday at New York.
"We're not abandoning Ricky Romero," Farrell said. "We're going to continue to work with him, we're going to continue to finish this year on a positive note and that is our intent going forward."
Millwood was perfect until Rasmus drew a one-out walk in the fourth. Encarnacion followed with a booming home run to left.
One out later, Adam Lind grounded a single to right, the final hit of the night for the Blue Jays.
Romero leads the AL in earned runs allowed (109) and his ERA of 5.87 is highest among qualified AL pitchers. ... Olivo is the sixth Mariners hitter with at least 10 home runs, the first time since 2007 Seattle has achieved that. ... The Blue Jays are 11-23 in one-run games. ... Toronto's Adeiny Hechavarria made his first career start at 2B, while Anthony Gose started in left field for the first time. ... Seattle ace Felix Hernandez (13-7) faces Venezuelan countryman Henderson Alvarez (8-12) in Thursday's series finale.