Locker took over late in the third quarter after Matt Hasselbeck went out with a sprained right elbow, having endured an ineffective day even before he was hurt.
Hasselbeck's status for next week's game against Tampa Bay is uncertain. He had ice on his arm after the game and said he expected to have tests on the elbow Monday.
"If Matt is fine, he's the quarterback," Munchak said. "There's no doubt about that."
Hasselbeck completed 13 of 25 passes for 124 yards with an interception before he was hurt when he was hit on his arm while trying to make a pass.
Locker took over for the next possession and on his third snap threw a 40-yard touchdown pass to Washington.
The Falcons were driving for the clinching score when running back Michael Turner had the ball knocked loose at the Tennessee 8 by Colin McCarthy. Will Witherspoon fell on the ball, and Locker guided Tennessee on a 14-play, 84-yard drive capped by a 4-yard scoring pass to Washington with 3:06 remaining.
The rookie made a 22-yard pass to tight end Jared Cook on a fourth-and-17, and he scrambled for 11 yards on a third-and-10.
Locker appeared in just two games before Sunday, completing one of two passes. This was his first real chance to show why he was a high first-round pick from Washington.
"It was a lot of fun," Locker said. "I would have liked a better outcome, but it was fun to be out there competing."
Locker said "it won't be a distraction to either of us" if there are more questions about who should start at quarterback for the Titans (5-5).
"Matt and I are good friends," Locker said. "We're close to each other, we want each other to have success."
Copyright 2011 by The Associated Press