What the hell is Mettenberger going to learn as a backup to a read option rookie? Absosmurfly nothing. Mett just needs to stick it out and then tell the Titans to go fuck themselves and sign with someone else in 2 years. His knee will be completely healed by then and will be 100 percent. Assuming Mariota isn't injured or benched by then.
He doesn't need to sit for 2 years. He needs to play and perfect his NFL QB skills which are quite different from the skills that get you by in college. Learn by doing, not by watching