Santos took the next step in his return prior to Toronto's Grapefruit League game against the Yankees by throwing a bullpen session.
"It was so weird kind of how it happened because I felt fine Sunday against the Phillies, finished the game and felt fine," Santos said of the injury. "I came here and did all my shoulder stuff and it was that night that I felt something in my triceps, like a soreness.
"I think it was just one of those things of gearing up to upper 20s in pitches and I was kind of throwing my sliders with a little more intensity. That might have been the cause, I still really don't know."
Santos threw all three of his pitches -- fastball, slider and changeup -- during his session on Sunday. He is now scheduled to pitch in a Minor League game on Wednesday afternoon before resuming his role in Grapefruit League action.
The injury appears minor but considering how last season went for Santos, there was initial cause for concern. Santos appeared in just six games for Toronto at the start of the season before being placed on the 15-day disabled list with soreness in his right shoulder.
That stint on the DL was supposed to be relatively brief, but the pain lingered and Santos eventually had to undergo season-ending surgery. The Blue Jays didn't want to take any chances this time around and sent Santos for an MRI after he informed the club about his sore arm.
"I think it was more so for them," Santos said of the MRI. "But I'm kind of glad I did it anyways because it took all the thinking out of my head because now I know there's nothing structurally wrong. They can see it and I knew it was just a matter of time of resting and the inflammation [subsiding]."
Santos' progression will be welcomed with open arms by the Blue Jays, who can ill afford to lose their hard thrower. Closer Casey Janssen is already somewhat questionable for the start of the season because of his rehab from offseason shoulder surgery.
Gregor Chisholm is a reporter for MLB.com. Read his blog, North of the Border, and follow him on Twitter @gregorMLB. This story was not subject to the approval of Major League Baseball or its clubs.