The team announced Wednesday that Arrieta was placed on the 10-day injured list due to the injury. Arrieta said he expects to undergo surgery in the next few weeks to remove the bone spur, plus any other loose bodies that could show up in an MRI scheduled for Thursday.
"I wanted to try and make it work for as long as possible, while I could remain effective and help the team," Arrieta told reporters. "I think after my start in San Francisco I realize that I'm not able to give the team what it needs. I'm confident and we're confident that we have options that can contribute to a further level than I was able to.
"The pain is something I can deal with, but it's the loss of feel and the ineffectiveness as the outings wear on. I think that the time is right to make it happen now and get a guy in the rotation that has pitched really well for us and give him an opportunity to get back to the form where he was at earlier in the season for us. I think we'll be in good hands."
Arrieta added that if he has surgery, he'll "probably" not pitch again this season.
Over his last 13 starts, Arrieta has a 5.76 ERA with 84 hits and 25 walks allowed across 65 2/3 innings. Zach Eflin will start in place of Arrieta on Saturday, and the Phillies are calling up first baseman Logan Morrison in a corresponding roster move.