Betances was making his first appearance of the season on Sunday, having just returned from the injured list, when he suffered the partial tear.
He faced two batters in the loss to the Blue Jays, striking them both out, then took a skip step off the mound. That’s when the injury appeared to happen.
“Heartbroken for Dellin,” said Boone, who called the injury “about as freak as can be.”
Boone said surgery is not being recommended at this time but could be an option down the line.
Betances, a 31-year-old right-hander, was diagnosed with an impingement in his pitching shoulder after his fourth spring training appearance on March 17. His rehabilitation stopped when the team said June 11 that he had strained his right latissimus dorsi muscle, and Betances did not start a minor league injury rehabilitation assignment until September.
He is eligible for free agency after the World Series.
The Associated Press contributed to this report.