BOSTON — The Red Sox’s rotation might have been dealt its first blow of 2014.

Felix Doubront exited Tuesday’s start against the Toronto Blue Jays at Fenway Park with left shoulder fatigue. Doubront had difficulty describing the discomfort after the Red Sox’s 7-4 loss.

“Right now, I feel fine. During the game, I couldn’t feel my shoulder,” Doubront said. “Weird. It felt weird. I don’t know if my whip or the release point was there. My arm speed wasn’t there. Nothing. I felt nothing.”

The left-hander will undergo an MRI on Wednesday.

It became clear in the fifth inning that something was physically wrong with Doubront, whose fastball velocity had begun to drop. Doubront threw a few grapefruits en route to surrendering back-to-back doubles to Jose Reyes and Melky Cabrera, which prompted a visit from manager John Farrell. Doubront was removed from the game at that point in favor of reliever Edward Mujica, who allowed both inherited runners to score.

The Red Sox won’t know until after Wednesday’s tests whether a disabled list stint is necessary. Clearly, it’s an alarming situation, though, as Doubront, who has dealt with shoulder fatigue in the past, said after Tuesday’s game he’s never felt anything quite like this.

“The more I threw, I started feeling it the second inning, when I walked (Dioner) Navarro,” said Doubront, who threw 65 pitches overall. “I noticed that I wasn’t feeling the grip in my hand. I started feeling that, the more I threw.”

The impact of Doubront’s shoulder issue was obvious, as he was knocked around by the Blue Jays early and often. All five of Toronto’s hits against Doubront went for extra bases, including a pair of homers clobbered by Edwin Encarnacion and Erik Kratz, and he was charged with five earned runs when all was said and done.

“The difference between his last two starts is when he made a mistake in the middle of the plate, they squared it up pretty good,” Farrell said. “And when he came out in the fifth inning, it was clear that the overall power to the stuff started to diminish. … They capitalized on mistakes once again, particularly with the home run.”

Catcher David Ross could tell something was up with his battery mate even before the southpaw walked off the mound in the fifth inning.

“(On) a couple of the hits, he threw an 85-mph heater, and I know that’s not him,” Ross said. “He was kind of cutting his fastball all day, which tells me he wasn’t trusting the release. He was getting away with it at first, but it came back to bite him as the game went on.”

Doubront has been inconsistent this season, but he entered Tuesday’s contest having pitched well in back-to-back starts. Now, the Red Sox need to cross their fingers that the shoulder issue that plagued Doubront against the Blue Jays isn’t an indication of something more serious.