Ortiz, who said he felt the discomfort after checking his swing on a 1-0 pitch from Blue Jays left-hander Rob Rasmussen with two outs in the ninth, was examined after the game, and a medicated patch was applied to the affected area.
Ortiz said he will be re-examined Friday, before the Red Sox open a three-game series against the Tampa Bay Rays in St. Petersburg, Florida. Until then, he said, he is not certain whether he will be available to play Friday night.
Ortiz, who hit four home runs in the first three games the Red Sox played here, said he first experienced discomfort in Wednesday night's 6-4 loss to the Blue Jays.
"On my last at-bat I kind of felt a little pop or something in my upper back," he said. "I felt a little bit of discomfort today, but I was fine [until] I made a rough move on that pitch -- tried to go and stopped -- and it pulled me a little more.
"I think it's a spasm. Hopefully, it's nothing serious."
Ortiz said he doesn't recall having issues in that area of his back before.
"They want to make sure tomorrow I don't have the same feeling," he said.
He also joked: "This is something that happens only to old people."
The Rays, winners of seven straight, have moved 2½ games ahead of the last-place Red Sox, who have lost three in a row after a five-game winning streak. This will be the first time the teams are meeting since Rays left-hander David Price hit Ortiz with a 94 mph fastball on May 30, the opener of a three-game series at Fenway Park.
"It's a war," Ortiz said after that game. "It's on. Next time I see [Price], he better put his gloves on. I have no respect for him anymore."
Price is scheduled to pitch against the Red Sox on Friday night. Asked if he anticipated any fallout from the last meeting, Ortiz said: "Why should I? I'm not the one who's pitching."