The coaches and the execs have to talk about Rush in a conservative manner. If they talk him up now and Rush regresses they will look pretty dumb. I know many around here think they are anyway. But that's another discussion.
But if they tamp down expectations and Rush continues to flourish, he will get the backup job. Linehan and Garrett can't ignore what is staring at them in the face. This is similar to when the coaches kept insisting that Romo would reclaim the starting job last year, even when Dak was tearing it up early in the regular season. Eventually, reality set in and Dak remained the starter. Same thing may happen with Rush.
They don't need to commit to a backup now. Just keep quiet and let the rest of the season play out. That's what they are doing.