Carreno made his first career start in the series finale against Cleveland and proceeded to allow four runs in six innings of work.
The cause for the demotion didn't have anything to do with Carreno's results on the mound but with Toronto not requiring a fifth starter until April 21, he was left on the outside looking in.
"We're afforded the ability to bring in another lefty to put in the bullpen," Blue Jays manager John Farrell said. "We'll go back to a four-man rotation for the time being ... we're looking forward to Aaron getting here."
Toronto is set to begin a three-game series against Boston following a grueling set against the Indians. In Cleveland, the Blue Jays needed 19 innings from their bullpen and as a result Farrell felt the club needed another rested arm.
That's where Laffey comes in. The veteran pitcher was a candidate for Toronto's starting rotation in Spring Training and will now pitch out of the bullpen.
Laffey surrendered seven runs in just 4 2/3 innings during his first start of the season for Triple-A Las Vegas but left a strong enough impression with Farrell that he earned the promotion.
"He had a rough outing there but the way he threw the ball in Spring Training, the way he threw against the Red Sox, particularly down in Fort Myers, Fla., one evening, it gives us a third left-hander with a very heavily loaded left-handed hitting lineup in Boston," Farrell said.
Laffey went 3-2 with a 3.88 ERA in 53 1/3 innings last season with the Yankees and Mariners.