Betances suffered the injury while jumping on the mound in Toronto, manager Aaron Boone said, according to Betelhem Ashame of MLB.com. He will miss the remainder of the season.
"(We’re) heartbroken for Dellin, knowing he worked so hard to get back to this point to be a factor for us and to have him successfully debut for us the other day and now this," Boone said.
Boone added that the injury occurred while Betances danced off the mound after recording the second out of the fourth inning, erroneously thinking the strikeout was the final out of the frame.
The 31-year-old will be treated conservatively. He's already received two opinions regarding the injury, and surgery remains an option.
The right-hander was on the injured list with shoulder and lat injuries before being reinstated for the team's postseason push. Betances faced a pair of batters in his 2019 debut against the Toronto Blue Jays and needed only eight pitches to strike out both before being removed from the game.
Over 358 career appearances, Betances owns a sterling 2.36 ERA and 2.31 FIP while notching 36 saves and 117 holds.
He's eligible to reach free agency for the first time in his career at the end of the 2019 season.