Manager John Farrell referred to Saturday's session as a "controlled bullpen" that saw the right-hander throw only fastballs.
Farrell said that it wasn't a maximum-effort session and that there is still no timetable for Santos' return, but he believes the fact that he is throwing off a mound is encouraging.
"You can't fast-forward," Farrell said when asked when Santos could be back. "The calendar might say one thing, but his body is going to be [saying] another thing. You are probably looking at three or four bullpens before he sees hitters."
Farrell doesn't expect Santos to faces batters for at least another week; from there Toronto will be able to get a better read on his status moving forward.
Santos hasn't pitched since April 20, landing on the disabled list with inflammation in his right shoulder.
The 28-year-old has appeared in six games, collecting two saves while blowing two and recording a 9.00 ERA.