"He's probably headed for surgery," Boone said. "Your heart goes out to Tommy, knowing it's a significant injury that has to get taken care of right now."
Kahnle underwent an MRI that revealed a a right ulnar collateral injury. The right-hander first felt right forearm tightness during the Yankees' impromptu workout Tuesday night at Camden Yards in Baltimore, following the postponement of their scheduled games against the Phillies in Philadelphia.
Kahnle, who turns 31 on Aug. 7, had an important role in the Yankees' bullpen, having thrown 61⅓ innings in 2019 with a 3.67 ERA and an average of 12.9 strikeouts for every 9 innings.
"It's a big loss," said outfielder Aaron Judge, who hit a two-run homer Friday in the Yankees' 5-1 victory over the Boston Red Sox in their rescheduled home opener. "During a game like this, when it's 3-1, you have that back end of our bullpen of Greeny [Chad Green], Tommy, Otto [Adam Ottavino] and then [Zack] Britton, that's tough to get by. Kind of missing a big piece like Tommy Kahnle is going to be tough for us."
Kahnle had pitched one scoreless inning this season, striking out three Washington Nationals batters in the eighth inning on Sunday.
"We're going to have to pick up the slack," Boone added. "Tommy is an elite reliever in the league; some guys are going to have to step up and pick up some important outs along the way."