It's always a given during the preseason that teams don't play their starters throughout the whole game. With that said, it's not unusual for a team to put out their starters on the first drive or two. This was a different story for Dallas though.
After all, we didn't see Russell Wilson play for the Broncos last night either, so why would we see Dak Prescott? There would be no point in starting Dak in any of these preseason games. This would not only be to protect him from injury, but also for him to understand his receivers.
When Prescott is on the sideline, he can see his players on offense perform from a different point of view. This gives him a better understanding of how his receivers and line function from the opposite team's view. After all, new receivers like Jalen Tolbert need to demonstrate to Dak how they run routes and such.
Prescott will most likely not be on the field in the next two preseason games. The coaches want him to stay healthy and maintain a focus on his goal, which is observing his team.
I never felt like installing your starters in the preseason was a smart move. The obvious reason is to avoid injury, but also to have your starter examine how not to play, and how to play. As we all saw, the Cowboys did not have a good game last night so this should be an example and lesson to the starters on what not to do during the regular season.
If the coaching staff wishes to continue this route, then we will not see Prescott play.