Kluber, the 35-year-old right-hander who has worked his way back from injury to earn his place as the No. 2 starter with New York behind Gerrit Cole, received a routine physical exam from team trainers Tuesday night, and he will be further evaluated Wednesday. An MRI will be a part of the evaluation, the team said.
"There is always concern when a starting pitcher comes out like that," Yankees manager Aaron Boone said. "His physical exam was pretty good tonight. Hopefully, we'll get some answers tomorrow, but it was just something he couldn't work through."
Kluber, pitching on five days' rest after tossing a no-hitter last week at the Texas Rangers, said his arm felt "kind of heavy" versus Toronto. He also told reporters in his postgame media availability that this feels different than last year's shoulder tear.
"It's frustrating," he said.
Kluber was bidding to match Johnny Vander Meer's feat of consecutive no-hitters, but that ended when Toronto's Bo Bichette reached on an infield single with one out in the third.
Kluber opened with 2⅓ hitless innings before Bichette grounded to shortstop and beat Gleyber Torres' off-line throw from the left-field grass. Vladimir Guerrero Jr. followed with a home run into the Yankees' bullpen in right-center on a flat cutter for a 2-0 lead. Kluber retired Randal Grichuk then was replaced by Michael King at the start of the fourth.
"He wasn't as sharp," Boone said. "And there was a handful of pitches where there were some misfires up."
A two-time American League Cy Young Award winner, Kluber threw 58 pitches, walked three and struck out five. His manager dismissed the notion that Kluber might have used too much energy last week en route to the no-hitter.
"He was going on his sixth day this time," Boone said. "He pitched a complete game last time, and he was pretty efficient. There wasn't a ton of stress or traffic there."
Kluber's 2019 season ended for the Cleveland Indians on May 1, when he was hit on the forearm by a comebacker; and his debut for Texas in 2020 lasted just one inning, before he tore a muscle in his right shoulder on July 26.
Kluber battled some control issues to start this season but was gaining momentum on New York's star-laden staff. Entering Tuesday, he was 4-2 with a 2.86 ERA and 50 strikeouts.
"We'll get our arms around it," Boone said, "and see where we're at."
Toronto's pitching was newsworthy for other reasons in the win. Steven Matz, a former Mets starter, returned to New York with his finest outing in two years, improving to 6-2. A 29-year-old left-hander from Stony Brook, New York, who won 11 games for the Mets in 2019 but faltered to an 0-5 record and 9.68 ERA last year, Matz gave up six hits in 6⅔ innings, struck out 10 and walked no Yankees.
New York didn't advance a runner past first until the seventh inning, when Clint Frazier doubled and Kyle Higashioka followed with an RBI single on Matz's 112th and final pitch -- his most since June 2019.
The Associated Press contributed to this report.