Kapler said the 33-year-old Arrieta recently told him about the trouble.
Earlier Sunday, a source had told ESPN's Buster Olney that the Phillies had been operating under the assumption that Arrieta could wait for surgery until after the season.
Arrieta has previously undergone surgery for the same issue, including a season-ending procedure in August 2011. He returned for the start of the 2012 season.
He was cagey about his health after Saturday's 6-5 loss to the New York Mets.
"Physically, I'm not in a great spot," he said. "I think we'll have a better idea over the next couple of days where I'm at."
Arrieta also hinted that he lacked a feel for his pitches during the outing.
It was a contentious outing for Arrieta, who hit three Mets batters, including Todd Frazier, then challenged the New York third baseman in his postgame comments.
"If Frazier's not happy about it, he can come see me and I'll put a dent in his skull," Arrieta said.
Frazier was unavailable for comment after the game, according to the Mets.
Arrieta underwent knee surgery early in 2019 but was ready to pitch by the start of the regular season. He is 8-7 with a 4.67 ERA in 18 starts this year.
He gave up a season-high 11 hits in the loss to the Mets.
"Overall I feel all right," Arrieta told reporters. "But we'll see. We'll see in the next couple of days."