Etling started the final seven games of his freshman season in 2013. After starting five games as a sophomore in 2014, he was supplanted by Austin Appleby.
Etling was in competition this spring to regain the starting job heading in the 2015 season, and coach Darrell Hazell had yet to release a depth chart.
The departure of Etling likely means Appleby will start and that David Blough, a redshirt freshman, will serve as the team's backup.
The Boilermakers finished last in the Big Ten in passing yards per attempt last season, but Appleby led Purdue to its first Big Ten win under Hazell in October.
Purdue opens the upcoming season on Sept. 6 at Marshall.