Haener starts vs Commanders: 1 INT, 5 punts ; Rattler replaced Haener in 2H: led to 19 points

He apparently practiced well enough to earn the #2 spot. So that's the call the coaches made. Rattler was far and away the better QB on the field today, so the choice is now pretty easy.
He was the #2 QB earlier in the year when Carr went down and Rattler got the call over him.