"I don't think he's going to make it," Frazier said. "It's going to be tough."
Even though the 41-year-old quarterback passed the preliminary tests administered to determine his recovery from a concussion suffered nearly two weeks ago, Frazier said Favre was unlikely to play.
Frazier declined to be specific about where Favre currently stands relative to the NFL's strict medical protocol for players after suffering concussions.
But Frazier said Favre -- officially listed as doubtful -- has not been medically cleared to play against the Lions and would be required to submit to further testing on the field in pregame warmups Sunday before he could participate.
If Favre can't go, rookie Joe Webb would make his second career start.