Of course, many believe that situation has already arrived -- as Rush has been great the past two weeks while Prescott has been out with a thumb injury.
Rush is 2-0 in his two starts, putting up QB ratings of 95.5 against the Bengals and 98.2 against the Giants -- leading some to think he's already done enough to keep Dak on the shelf even after the Week 1 starter gets healthy.
But Johnston told us he doesn't see that happening at all ... explaining he thinks this is an entirely different situation than the one Dak had back in 2016, when he formally took over for an injured Tony Romo.
The former Cowboys fullback, though, believes Rush -- an impending free agent -- has made some money for himself regardless ... showing he can be one of the premier backup QBs in the league going forward.
Moose also spoke with us about the Giants vs. Bears game he's broadcasting this weekend for FOX -- saying he's excited to see how both Daniel Jones and Justin Fields play in their fourth games of the season.