"David has been treated and evaluated by our medical staff daily for a recurrence of concussion symptoms and also visited with and received feedback from specialists," said Texas' head athletic trainer Kenny Boyd in a release. "Though he's made a lot of progress, we have not been able to clear him to return to competition. Due to the duration of symptoms, we are now at a point that we all believe the best approach for him is to not return this season. We've visited with David and his family and are all in agreement on his not returning to action as the best next step in his recovery. We will continue to treat him and watch his progress closely and expect this will allow him ample time to return to full health and participation for the spring season."
Ash originally suffered the concussion in the second game of the season against BYU. After missing the Ole Miss game the following week he returned for the Kansas State game, but did not finish the game and hasn't played since.
And given the obvious severity of the concussion, it really doesn't make any sense to bring Ash back now. Ash is a junior and still has one season of eligibility remaining.