Threet, who will be a senior in 2011, has suffered four concussions, two of which occurred last season. Threet told The Arizona Republic on Wednesday that he's still experiencing symptoms from the last concussion and still has headaches and has trouble sleeping.
Threet had a concussion Oct. 23 at California and another Nov. 26 against UCLA that ended his junior season.
"Doctors have told him a fifth concussion could be debilitating," a source told ESPN. "You don't want to be 35 years old and can't hold up a cup of coffee."
Last season, Threet completed 62 percent of his passes, throwing for 2,534 yards with 18 touchdowns and 16 interceptions for the Sun Devils. He was sacked 23 times for ASU (6-6). It was his first season of play since transferring from Michigan, where he was the Wolverines' starting quarterback in 2008.
The 6-5, 240-pound Threet, who is on schedule to graduate in December, will likely remain close to the program and help as a student assistant and work with the QBs, the source said.
Arizona State begins spring football in the last week of March. Rising junior Brock Osweiler, who started the final two games of the 2010 season (both Sun Devils wins), is expected to be the team's No. 1 quarterback.
Bruce Feldman is a senior writer for ESPN The Magazine. Information from The Associated Press was used in this repor.t