The Toronto Blue Jays will need to wait until next season to see if Sergio Santos is the closer they hope he is.

Santos will undergo season-ending shoulder surgery after he felt discomfort during a throwing session Saturday. The 29-year-old right hander is anticipated to return to the team for spring training, although a timeline won't be established until after the surgery is completed.

Blue Jays manager John Farrell said Santos had tried to work through the injury without surgery, but it became apparent there was no other option.

"Given all that he's gone through, taking every step along the way, we're at the point where it's got to be surgically explored, repaired. That's where we're at," said Farrell on Sunday.

The date for the surgery has not been scheduled. Farrell said they believe the injury is to Santos' labrum, and that his rotator cuff appears OK.

As it became clear that Santos was taking longer than expected to recover, the Blue Jays put him on strength and conditioning training to try to remedy the shoulder.

"As he got back into his throwing program, flat ground long toss, he felt good in that regard," said Farrell. "But when he gets on the mound with that increased intensity that's where he feels the discomfort. So that's where the surgery's the next step."

The news will be a disappointment to the team, which hoped they had a permanent closer when Santos was acquired in a trade with the Chicago White Sox in December. But the six-foot-three, 240-pound pitcher made just six appearances in April and had only two saves before being put on the disabled list following a game in Kansas City.

Farrell said the team doesn't believe Santos was injured when they acquired him, and that he felt pain in the shoulder after his outing against the Royals on April 20. Casey Janssen has since taken over the closer role, converting 13-of-14 save opportunities this season.

Santos spent two years in the majors with the White Sox prior to coming to Toronto. He has 33 career saves, including 30 last season, and 3.53 earned-run average.