Wang served up a two-run home run to Adam Lind in the frame, giving the Blue Jays a 4-3 lead at the time, and threw one pitch to Scott Rolen -- a ball -- before catcher Jorge Posada saw something in Wang's delivery and trotted out to the mound.
It was the velocity of the pitch that tipped the Yankees off, manager Joe Girardi said. Wang had felt tightness when he surrendered the home run and experienced pain on the pitch to Rolen, with his sinker dropping in velocity to 86 mph.
"We actually thought it was a slider from the bench, and that's when Jorgie called us out," Girardi said.
Upon reaching Wang, Posada gestured to the bench, calling out Girardi and assistant trainer Steve Donohue.
Wang walked off the field and was replaced by Dave Robertson, who had been warming already in the bullpen, but was given unlimited time to get ready on the field because of the injury.
The Yankees will need a starting pitcher on Thursday. Girardi said that Phil Hughes and Alfredo Aceves are not stretched out enough to pitch a full outing, but Aceves could offer 55 to 60 pitches and Hughes 45 to 50.
Yankees general manager Brian Cashman had floated the idea this week of a start-by-committee in the case of an emergency, though the club is also high on Sergio Mitre, who is 2-1 with a 3.26 ERA in five starts at Triple-A Scranton/Wilkes-Barre. In 30 1/3 innings, Mitre has walked three and struck out 23.
Thirty minutes after leaving the field, Wang was on his way to Columbia Presbyterian Hospital in Manhattan for the MRI exam. He made it back to Yankee Stadium in the 11th inning of the Yankees' 6-5 walk-off victory, but not bearing good news.
"It's definitely frustrating," Wang said. "Hopefully, I can get back to the field soon and contribute to the team."
An assignment to the DL would be the third within the span of a little over a calendar year for Wang, who suffered a season-ending right Lisfranc injury on June 15 of last year at Houston while running the bases.
He was also shelved from April 24 to May 22 this year with weakness in the abductor muscles of both hips, missing 26 games.
"It's tough. When he's healthy, he's a great pitcher," Yankees first baseman Mark Teixeira said. "That's what he's been his entire career. He's been battling injuries this year, and we just need to get him healthy."
According to Wang, the sensation is similar -- but not identical -- to the discomfort he felt when he battled shoulder injuries in both 2001 and '05.
Wang missed the entire 2001 Minor League season recovering from arthroscopic surgery on his right shoulder performed in April of that year and missed 52 games when he was on the disabled list from July 13-Sept. 6, 2005, with inflammation in his right shoulder.
"Back in '05, when I stretched my arm backwards, it hurt," Wang said. "This is different."
He allowed four runs on six hits in 5 1/3 innings to Toronto, walking one and striking out one as he lowered his ERA to 9.64.
"I'm confident he'll be back. I just can't tell you exactly when," Girardi said. "It's been a trying year for him already, what he's went through. Today was probably his best start, and now we have to deal with this."