The 31-year-old is earning $3.75 million this season and has three seasons of club options remaining on his contract, which can be bought out for $750,000. He has been removed from Toronto's 40-man roster and is now expected to report to the Bisons.
Santos battled some control problems this year and appeared to lose the trust of manager John Gibbons. He's walked 17 batters and has a 7.78 ERA in 19 2/3 innings, picking up five saves in 24 appearances.
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.