Stafford struggled in the second half of the season as he threw 13 of his 19 interceptions in his side's last eight games, which cost them a place in the playoffs as they fell from 6-3 to end the year 7-9.
Mayhew told MLive.com: "I think there certainly has to be some work done. I couldn't really quantify that. I don't think his confidence has fallen very far.
"I mean, think about it. In Week 9, his numbers were outstanding. We were winning games. We were playing really well.
"Didn't finish up the second half of the season the way we expected to, do the things that we wanted to do. But there are a lot of moving pieces to that. It's not all on Matthew."