Morrow didn't sound too concerned about the injury when speaking after Toronto's 2-0 loss, but he said the pain started Wednesday -- the day after a between-starts bullpen session -- then crept up again while he was warming up in the bullpen at Osceola County Stadium.
Morrow will rest for a couple of days, but besides that, the plan for him moving forward is unknown. But Morrow said he's dealt with these short bouts with shoulder soreness the past couple of Spring Trainings and isn't concerned that it will linger.
"It's always been a thing that's kind of popped up and then just gone away and really hasn't bothered me after I've gotten it to calm down," Morrow said. "It's kind of a getting-over-the-hill thing."
Acquired from the Mariners in December for Brandon League and Johermyn Chavez, Morrow posted a 3.60 ERA in 10 starts for Seattle's Triple-A affiliate in 2009, then went 2-4 with a 4.39 ERA in 26 games (10 starts) for the Mariners.
Morrow, who went on the 15-day disabled list last May with right biceps tendinitis, is expected to be a key piece of Toronto's rotation this year.
"We just weren't going to take any chances with him," Blue Jays manager Cito Gaston said. "We think he's going to be a big part of this organization, and we just weren't going to chance it.
"[We'll] play it safe, get him re-evaluated and see how we go from there. Well see how he feels in a couple of days and perhaps get him back to a doctor [in Dunedin, Fla.,] too."
Alden Gonzalez is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.