Coke is 1-8 with a 4.91 ERA after allowing seven earned runs in four-plus innings against New York on Wednesday night.
"I'm giving everything I've got, and I'm coming up short -- period," Coke said.
Coke was a reliever for his first three seasons in the majors. Detroit moved the left-hander to the rotation this season.
He had tough luck at times. Coke threw 6 1/3 scoreless innings in a no-decision June 13 against the Tampa Bay Rays -- but in his last three starts, he allowed 16 earned runs in 13 2/3 innings.
"We had hoped that it would work out better," manager Jim Leyland said. "We think the stuff's there."
Furbush, a rookie, has made 12 appearances, all in relief. The left-hander is 1-1 with a 2.49 ERA.
Detroit lost 14-3 to the Mets on Tuesday and 16-9 on Wednesday. The Tigers sent Justin Verlander to the mound Thursday, hoping for a reprieve for a battered bullpen.
Coke takes Furbush's spot in the bullpen.
"I'm ready to go today, if asked," Coke said Thursday. "I just had my job title taken from me and given a different one. OK, so what? Give me the ball and get out of the way. Let's roll."
After going from relieving to starting and now back, Coke said he doesn't want to hear anything more about the transition between the two roles.
"Everything's been all about this transition," he said. "It's all you guys have harped on. I'm tired of hearing about it. I'm sick of it. Maybe I have to transition back to just walking out to the bullpen. Oh my god, that's going to be hard."