"The past four starts it's been about fourth, fifth inning I start to feel it," Cosart said. "Today it was pretty bad in the first. ... It got to the point where I felt like I needed to come out."
Cosart will be re-evaluated Sunday and should have an MRI on Monday to see the extent of his elbow injury and make sure it's nothing more than the bone spur he's been dealing with. Padres manager Andy Green said the team had been monitoring the issue and they felt good about his health entering the game.
The 26-year-old was scheduled to throw one more start this season. Cosart said making another start at this point in the season might not be the best idea, although he added he wouldn't be opposed if athletic trainers and doctors think there would be a way to subside the pain.
"It's to the point now where it's something I'm kind of tired of dealing with," he said. "... We're hoping it's just a bone spur. That's a simple procedure: Go in there and scope it, get it cleaned out and it will be a non-issue.
"That's positive thinking, that's what we're hoping for, but it's up to the trainers and the doctors. ... But it's to the point now where I'm sacrificing command, I'm sacrificing execution of pitches and that's really only going to hurt my team having to go to the bullpen early in games. The goal is to get it figured out overall in the next couple days and go from there."
The Giants scored three runs in the first after Cosart struggled to find the strike zone -- just 16 of his 33 pitches were strikes -- and gave up singles to Buster Posey, Joe Panik and Angel Pagan.
Left-hander Keith Hessler stepped on the mound to start the second in relief of Cosart. Hessler has a 4.43 ERA this season through 20 1/3 innings with Arizona and San Diego.
Carlos Collazo is a reporter for MLB.com based in San Diego. Follow him on Twitter @CarlosACollazo. This story was not subject to the approval of Major League Baseball or its clubs.