Santos was placed on the 15-day disabled list April 14 with soreness in his triceps. He had been close to starting a minor league rehabilitation assignment but experienced a drop in velocity while pitching in an extended spring training game last Saturday, prompting another examination.
Santos is 0-1 with 2.08 ERA in five games this season. He was limited to six games last year because of shoulder surgery.
Shortstop Jose Reyes, out since April 12 with a severely sprained left ankle, is out of his walking boot and will soon be able to start light jogging. While Reyes initially was expected to be out until the All-Star break, general manager Alex Anthopoulos said the four-time All-Star might be able to return by the end of June.
Reyes was batting .395 with one homer and five RBIs in 10 games before getting hurt on an awkward slide into second on a stolen base.
Toronto also said Tuesday that outfielder Melky Cabrera will undergo an MRI on Wednesday to determine the cause of persistent soreness in his legs, particularly his left hamstring.
Cabrera, who signed a $16 million, two-year contract in November, has been bothered by sore legs since the end of spring training, according to Anthopoulos. Cabrera is batting .261 with one homer and 12 RBIs and has played in every game this season for the Blue Jays.
Anthopoulos said that right-hander Brandon Morrow, who has not pitched since May 5 because of a sore neck, will miss another turn in the rotation. Right-hander Ramon Ortiz will start in Morrow's place against San Francisco on Wednesday, with Morrow scheduled to pitch Saturday at the New York Yankees.
Morrow, 1-2 with a 4.69 ERA in seven starts, was first pushed back and then skipped entirely from a scheduled start at Boston last week.
Right-hander Josh Johnson, on the disabled list since April 29 with a sore triceps, is throwing at 120 feet on flat ground and is set to pitch off a mound. Acquired from Miami in November, Johnson is 0-1 with a 6.86 ERA in four starts.