Mariota missed the entire week of practice while dealing with a sprained MCL suffered in Week 6.
"He's frustrated because he wants to play," Whinsenhunt said of Mariota, per the team's official twitter account.
Mariota was optimistic earlier this week that he could play, but as NFL Media's Rand Getlin noted, per a source close to Mariota, the Titans know they have to protect the rookie from himself, given how competitive he is.
Mariota started the season hot, with a 158.3 passer rating in Week 1, but has struggled in recent weeks as defenses have adjusted to the rookie's tendencies. He posted just a 67.6 passer rating and was clearly hobbled after the early hit last week.
Mettenberger is a capable backup. The second-year signal caller boasts a big arm, but struggled at times last season with slow reads and questionable throws into traffic.
"I'm excited about the opportunity," Mettenberger told reporters Friday after he was announced the starter.
Mettenberger posted a 59.8 completion percentage with 201.7 passing yards per game, eight touchdowns, seven interception and a 83.4 passer rating as a rookie. The Titans lost all six games he started last season.