The veteran right-hander is scheduled for an arthrogram on his right elbow on Thursday, and the club will determine which steps to take from there.
For his part, Arrieta isn't optimistic about his chances of returning to the mound this campaign.
"I'll probably miss the remainder of the season," Arrieta said, according to Mark Gonzales of The Chicago Tribune.
Arrieta revealed in July he was dealing with a bone spur in his elbow. While surgery was deemed an eventual likelihood, he elected to try to pitch through the injury.
The 2015 NL Cy Young winner last pitched on Sunday, allowing five runs on seven hits through three innings of work.
After the start, Arrieta told Jim Salisbury of NBC Sports Philadelphia he and the club were considering the possibility of ending his season.
Arrieta is 8-8 with a 4.64 ERA and 1.47 WHIP through 24 starts this season. He's in the second year of a three-year, $75-million contract that also includes team options for 2021 and 2022.
While the Phillies haven't made a public announcement, Arrieta's name was not listed on the team's lineup card, Montemurro noted. Infielder Logan Morrison is there in his place, who the club has apparently called up from Triple-A.