ANAHEIM -- Even Shohei Ohtani sometimes turns in a tough outing. But it could’ve been due to some back stiffness he experienced after throwing a slider to fellow superstar Vladimir Guerrero Jr. in the first inning on Thursday night.

The two-way star remained in the game despite the injury and struck out 10 over six innings, but he gave up a lot of hard contact, including two homers and five runs in a 6-3 loss to the Blue Jays in the series opener at Angel Stadium. Ohtani’s velocity was down a tick -- his fastball velo was 1.6 mph lower than his season average -- and he believes it could have been due to his back. Ohtani said he believes it’s a minor injury, though he won’t know more until being further evaluated on Friday.

"As of now, I don't think it's a big issue,” Ohtani said through interpreter Ippei Mizuhara. “But we'll have to wait until [Friday] to see how it feels. I'd definitely say my back [caused the velocity decrease]. I was able to get a bunch of strikeouts, but I wasn't pitching at my full strength."

• Right this way! Ohtani directs runner back to dugout

Angels manager Joe Maddon also didn’t seem too worried about the injury, but he pinch-hit Taylor Ward for Ohtani in the eighth inning to give Ohtani a rest. It was the first at-bat since May 20 for Ward, who has been out with a neck stinger that has caused weakness in his right throwing shoulder. There’s a chance Ward could serve as the designated hitter on Friday if Ohtani is ruled out.

"He has a stiff back, but it's nothing serious, so I didn't want to push him right there,” Maddon said. “I'm not picking up on fatigue. Nobody's perfect. He just did not have one of his better nights."

Ohtani, who allowed five runs on six hits and a walk to fall to 3-3 with a 3.45 ERA, also didn’t think fatigue was a factor, even though he’s been a constant again for the Angels this season. He’s played in a team-leading 45 of the club’s 46 games and is also tied for the team lead with eight starts on the mound.

"It's hard to say, but since I felt it on one certain pitch, I'm thinking it's not really fatigue,” Ohtani said. “It might have been something to do with my mechanics."

Ohtani and the Halos were trailing even before he felt the stiffness in his back. The right-hander gave up a leadoff homer to George Springer on the eighth pitch of the game, which was a 3-2 fastball. Ohtani retired the next six Toronto batters, but he ran into trouble in the third.

Ohtani walked Lourdes Gurriel Jr. to open the inning before giving up back-to-back singles to Springer and Santiago Espinal with one out. Espinal's knock plated a run before Bo Bichette brought home two more with a double.

"I just gave up hits in big spots with runners on base,” Ohtani said. “That pretty much sums up my day today."

Ohtani again settled down -- and was helped by a leaping catch at the left-field wall by Brandon Marsh to rob Danny Jansen of extra bases in the fourth -- but he served up a homer to Guerrero to open the sixth. Ohtani, who won last year's American League MVP Award with Guerrero finishing second in voting, hung a 1-1 curveball that Guerrero hit off the foul pole in left field.

"We're in the same league but different divisions so there's not a lot of chance to face him but he's a great hitter,” Ohtani said. “I take away from great hitters, whether they're lefty or righty. So that home run I gave up, I can learn something from it."

It wasn’t all bad for Ohtani, however, as he reached the 10-strikeout mark for the third time this season. He recorded 17 swing-and-misses -- seven with his splitter, four with his slider, three with his cutter, two with his curveball and one with his fastball. He was removed after 93 pitches, as he still hasn't thrown more than 99 in a start this year.

Maddon said that while Ohtani showed some flashes, he just didn’t put it all together in this outing.

"He just wasn't as sharp as he can be,” Maddon said. “The velocity was not that high of a number. His splitter was great sometimes, but other times, not too good. None of his pitches were consistent."