Kapler had to be carted off the field after being injured while running around the bases during the fifth inning of the Red Sox's 5-3 win over the Blue Jays at the Rogers Centre. Toronto's team orthopedist determined that Kapler had ruptured his left Achilles tendon.
The Boston outfielder, who had been filling in for injured center fielder Johnny Damon, had a splint attached to back of his leg and was walking around on crutches after the game. When the team arrives back in Boston, Kapler will have his foot examined by team doctors to evaluate the extent of the damage.
"I just haven't really had a chance to allow it to sink in," Kapler said after the game. "There's going to come a time when reality hits me. It's going to be a while before I can help my teammates. That's going to be hard for me."
With Kapler on first base, second baseman Tony Graffanino pulled a pitch from Toronto's Josh Towers deep to left field. The ball was heading toward the left-field wall and Kapler began to sprint around the bases.
Graffanino's hit landed in the bullpen for a two-run home run, but Kapler's trip around the bases ended abruptly a few steps beyond second base. Kapler stumbled and fell face first into the turf and laid motionless on the field for several minutes.
At first, Boston manager Terry Francona stayed in the dugout, thinking that Kapler had just tripped.
"I got to the top of the dugout when he went down and I didn't go out there," Francona explained. "I thought, 'He's going to get up, we're all going to laugh.' That didn't happen. The second I got out there, he looked up and said, 'It's not good.'"
Graffanino stood on second base while Kapler remained down about halfway to third base, while Boston trainer Jim Rowe and Francona assisted him.
"What he relayed to [Rowe] is it felt like a baseball hit him in that area," Francona said. "Evidently, that is exactly what everybody says that has ever had it."
Kapler said that he felt a pop in the back of his ankle, but that it did not cause any serious pain. He said didn't "feel anything" in the foot after the game.
"One thing I know is that Gabe is a tough guy," said David Ortiz, who hit the game-deciding home run on Wednesday. "He's a guy that is not afraid of pain. So when I see everybody running on the field to get him up, I knew there was something wrong."
"You could categorize him as 'Charlie Hustle,'" said David Wells, who picked up the win on Wednesday. "I've never seen that happen in all my 18 years."
Red Sox infielder Alejandro Machado came in as a pinch-runner and continued Kapler's run around the bases to score the first run of his Major League career. Machado remained in the game as Boston's center fielder.
According to baseball's rule book, "if an accident to a runner is such as to prevent him from proceeding to a base to which he is entitled, as on a home run hit out of the playing field, or an award of one or more bases, a substitute runner shall be permitted to complete the play."
After Kapler was helped to his feet by team trainers, he asked if he could finish the walk around the bases with their help.
"It's embarrassing to be out there and not be able to finish it," Kapler said. "So, I would've liked to be able to at least run around the bases. But they told me that wasn't the right thing to do."
Damon, who has been back in Boston to rest his ailing left shoulder, is rejoining the Red Sox on Thursday for their four-game series against Oakland at Fenway Park. Just in case he is not available, though, Boston will call up outfielder Adam Hyzdu from Triple-A Pawtucket.
"I'm really hoping that Johnny Damon shows up and is really healthy," Francona said. "It's not good to get banged up, but I think we all view it as, 'We're the Red Sox and we're going to do it together.'
"I believe it, they believe it, and when that gets tested, I think we're at our best."
Jordan Bastian is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.