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TORONTO -- John Lackey grabbed at his arm in pain, fearful that another long rehab might lie ahead of him.

For now, the Boston Red Sox are hoping that won't be necessary.

Lackey left with a strained right biceps in his first start since September 2011, coming out in the fifth inning Saturday as the Red Sox lost to the Toronto Blue Jays 5-0.

"It's all in the biceps, it wasn't the elbow," Lackey said. "Pretty frustrating. A little scary when it happened, for sure."

Lackey (0-1) missed last season following elbow ligament-replacement surgery in November 2011. He allowed a two-run homer to J.P. Arencibia in the fourth, then clutched his arm and jumped around the mound in pain after missing low and inside to Jose Reyes on a 2-2 pitch with one out in the fifth.

"We called cutter away and I knew something had to be wrong," catcher David Ross said. "He just jerked it down and in. I turned around to get a ball from the umpire and saw him grabbing his arm."

Boston manager John Farrell and several teammates came to the mound before Lackey walked off with trainer Rick Jameyson and was replaced by Alfredo Aceves, who threw ball four.

Lackey was asked what went through his mind after his arm tightened up.

"A year and a half of getting to this point and having to do it again," he said.

Early results were more encouraging, though. Farrell said Lackey's arm loosened up after icing, and tests didn't indicate any lack of strength.

"My hand didn't go numb or anything like that, like I've had with elbow stuff," said Lackey, who will fly to Boston on Sunday morning to undergo an MRI.

Farrell said it was too early to say who might pitch if Lackey is unable to make his next start.

"We've just got to get every test and run him through the appropriate exam," Farrell said. "We'll put our heads together once we get all that information."

Lackey signed an $82.5 million, five-year contract with Boston in December 2009 after eight seasons with the Angels. He allowed two runs and five hits in 4 1/3 innings with eight strikeouts and one walk Saturday, throwing 50 of 76 pitches for strikes and reaching 94 mph.

"He looked sharp," Farrell said. "He had good action to his secondary stuff, the velocity was probably the best we've seen to date. He was free and easy, he threw all of his pitches for strikes."

Farrell called the biceps muscle "an abnormal area" for a pitcher to feel cramping.

Colby Rasmus hit a three-run homer in the sixth off Aceves, a drive off the facing of the third deck in right-center.

J.A. Happ (1-0) combined on a two-hitter with Steve Delabar, Aaron Loup and Sergio Santos.

After Jacoby Ellsbury's leadoff double in the first, the Red Sox didn't get another hit until Dustin Pedroia reached on an infield single with two outs in the ninth. Mike Napoli followed with a drive to deep center, but Rasmus made a fine running catch before crashing into the wall.

Pedro Ciriaco drew a one-out walk in the second, advanced on a wild pitch and went to third on a throwing error by Arencibia, then was thrown out at the plate by first baseman Edwin Encarnacion while trying to score on Ellsbury's grounder.

Happ struck out six and walked three in 5 1/3 innings.

"Happ was dynamite," Blue Jays manager John Gibbons said. "He started throwing his changeup more and that was a big pitch for him."

Toronto was without slugger Jose Bautista for the second straight day because of a sore right ankle. The two-time major league home run leader was hurt stepping on first base while beating out a double play Thursday. Bautista hit in the cage and ran on the field before the game, and was available to pinch hit.

Adam Lind walked to begin the sixth and stole second, just the sixth stolen base of his career. Mark DeRosa drew a one-out walk before Rasmus clobbered a three-run shot off the facing of the third deck in right-center, his second.

Game notes

The Red Sox were shut out for the first time since Sept. 16, 2012, when they also lost 5-0 to Toronto. ... Attendance was 45,797. ... Toronto selected the contract of RHP Dave Bush from Triple-A Buffalo and designated RHP Jeremy Jeffress for assignment. Jeffress made his first appearance of the season in Friday's loss to Boston, giving up a solo homer to his first batter, Will Middlebrooks. ... Red Sox SS Jose Iglesias, who left Friday's game two innings after being hit on the arm by a pitch, was held out of the starting lineup. Farrell said it was a scheduled day off. ... Bench coach Torey Lovullo, not Farrell, brought the lineup card to home plate before the game. Farrell, who managed the Blue Jays the past two seasons before leaving to replace Bobby Valentine in Boston, was booed throughout the series opener Friday night.