Phillips was the No. 1-ranked defensive recruit (No. 3 overall) in the country in the ESPN 300 Class of 2017, but he appeared in only 11 games for UCLA.
He made 20 tackles over the first four games of this season, including nine with a sack against Cincinnati, before a concussion ended his season in late September.
Bruin Report Online reported there is a chance Phillips could return to football if he is eventually cleared to play, but in such a case he would be likely to transfer.
ESPN's Kyle Bonagura contributed to this report.