Gabbert kept the Titans' playoff hopes alive.
"I don't know," Gabbert said when asked if this was the biggest win of his eight-year career with four teams. "It was a fun win. We got a big game next week."
The victory was Tennessee's first in the seven career games that Mariota has left with an injury.
Coming off the bench for the third time this season, Gabbert threw for 101 yards after the Redskins knocked Mariota out of the game late in the first half with a stinger. Gabbert hit Taywan Taylor for 35 yards to jump-start the winning drive, Derrick Henry ran four times for 33 yards, his last an 18-yarder to the 2. Gabbert then hit Pruitt at the back of the end zone.
"The only scenario that any of us are smart enough to worry about is us preparing to win and ultimately win, and that's all we can do right now," first-year Titans coach Mike Vrabel said.
The Redskins (7-8) have lost five of six, and coach Jay Gruden said they couldn't ask for anything more from quarterback Josh Johnson, who came off his couch only weeks ago. Washington needs losses by Minnesota and Seattle to avoid being eliminated.
"I'm just very proud and sick for the way it ended for him tonight because he really just played (great), displayed great courage, leadership all in three weeks," Gruden said. "I hate for him to be judged on that one pass (Byard's interception), but overall just very, very proud of the way he came in here and led this team."
Mariota had thrown for 110 yards when sacked with 48 seconds left in the first half by defensive end Jonathan Allen. His right, throwing shoulder and arm were examined, then he walked to the locker room and was replaced by Gabbert.
The stinger is the same injury that knocked him out of Tennessee's loss Nov. 18 at Indianapolis. He also was knocked out of the season opener with an elbow injury that cost him a start. Vrabel said Mariota was still being evaluated after the game.
Byard said the Titans have full trust in Gabbert.
"It's really no panic on the sideline," Byard said.
Henry finished with 84 yards and a TD on 21 carries.
Johnson threw for 153 yards and a TD. Dustin Hopkins kicked field goals of 50, 40 and 46 yards, and Washington finished with three sacks.
The fourth quarterback to start this season for Washington this season, Johnson drove the Redskins on the NFL's second-longest drive this season, taking the ball with 26 seconds left in the first quarter and then eating up 10:58 off the clock, going 93 yards over 17 plays. Johnson capped the drive with a 7-yard TD pass to Michael Floyd for a 10-6 lead.
Titans Pro Bowl defensive tackle Jurrell Casey aggravated a knee that kept him out of practice all week with 4:21 left.
Adrian Peterson ran for 119 yards for Washington. That put him over 1,000 yards for the season for the first time since 2015 and made him the 12th player in NFL history with eight or more seasons with 1,000 yards rushing. None of that mattered with the loss.
"Hopefully, I'll appreciate it after Christmas, but for the time being, you know, I play this game to win and to have an opportunity to win a championship," Peterson said. "Falling short of having that opportunity is weighing heavy on my heart right now. I'm just going to let this one sink in and bounce back and finish the season off on the right note."
Redskins: Host Philadelphia.
Titans: Host Indianapolis.
Follow Teresa M. Walker at www.twitter.com/teresamwalker
More AP NFL: https://apnews.com/NFL and https://twitter.com/AP-NFL
Canadian Football League, players' union reach tentative labor deal, ending strike
The Canadian Football League has reached a tentative agreement with its players' union on a new labor contract, ending the second strike in CFL history.
Los Angeles Chargers hold ceremonial groundbreaking on new headquarters in El Segundo
The Los Angeles Chargers held a ceremonial groundbreaking Wednesday on the team's new headquarters in El Segundo.
Allen, Beane talk about the importance of supporting the Buffalo community
Josh Allen and Bills GM Brandon Beane open up about volunteering and supporting the Buffalo community in the wake of the mass shooting at Tops Friendly Markets.
Buffalo Bills players, coaches, front-office staff volunteer to support community in wake of shooting
Bills players, coaches, front-office members and other staff members visited the scene of Saturday's mass shooting on Wednesday and dished out food and handed out groceries to show their support for the community.
Los Angeles Chargers sign defensive lineman Morgan Fox
The Chargers continued the offseason rebuild of their defense by signing defensive lineman Morgan Fox on Wednesday.
Minnesota Vikings assistant HC Mike Pettine organizes coaching diversity summit for entry-level candidates
Minnesota Vikings assistant head coach Mike Pettine has organized a coaching diversity summit this week at team headquarters, a program designed to increase the diverse candidate pool for entry-level NFL jobs.