The series is to move to Saturdays at 8 p.m. ET when the show starts up its sixth season Oct. 8.
TVGuide.com said Warburton blasted the move during the weekend at the roast for troubled star Charlie Sheen.
"What a (expletive) time slot," Warburton told TV Guide Magazine at the event.
"The scripts are great and we're having a lot of fun doing the show, but God knows if anybody's gonna find us on Saturday night," he said. "It seems like they're just keeping us mid-season once again. ... They'll run us on Saturdays and if they've got a slot that opens up in the week, they'll put us back out. But nobody's excited about Saturday night. ... This is probably our last year of the show. ... The show does really well in reruns, and if they want to syndicate, they need to get to 100 episodes. So they're doing that to us."