Santos, who is working out at the Blue Jays' Minor League complex in Dunedin, Fla., played catch from 90 feet on Monday and is expected to be stretched out to 120-150 feet in the coming days.
Toronto's closer has been out since April 21 because of inflammation in his shoulder. There's no timetable for his return, and Santos is expected to go on an official rehab assignment before being activated from the 15-day disabled list.
"I think it's still too premature because if he advances through the long-toss phase, it can shorten it," manager John Farrell said. "If we get to the mound sessions and he needs to repeat a step, I don't think we have a date earmarked yet as far as his return."
Injured starter Dustin McGowan also has resumed throwing following a right shoulder injury. McGowan felt some discomfort in the area while working his way back from an unrelated foot injury and was immediately shut down from all baseball activities.
McGowan has started throwing catch slightly ahead of schedule, but is not close to going on a rehab assignment.