The Mariners transferred Santos (lat) from the 15-day injured list to the 60-day IL on Thursday.
The transaction was made in order to free up a spot on the 40-man roster for infielder Leonardo Rivas, whose contract was selected from Triple-A Tacoma. Santos is in the midst of a throwing program as he works his way back from a right lat strain, but the Mariners are proceeding conservatively with him. He won't be eligible to make his season debut until late May.