Pachall left the school after an October arrest for suspicion of driving while intoxicated, the Fort Worth (Texas) Star-Telegram reported Monday.
"He looks great. He did everything we wanted him to do," said coach Gary Patterson, adding Pachall attended a team meeting Sunday.
Stan Pachall expressed appreciation for the support his son received, saying in an email, "To the people that support and have prayed for Casey's recovery, thank you."
Pachall will begin workouts with the team and will be given an opportunity to earn the starting quarterback job, the newspaper said.