This happens to be the fifth torn ACL in seven games for the Chargers this season.
San Diego will likely place Reid on injured reserve in the coming days and add a player to their active roster.
Reid, 24, is a former fifth-round pick of the Lions back in 2014. He lasted just over two years in Detroit before he was waived coming out of the preseason and later claimed off of waivers by the Chargers.
Reid is in the third year of his four-year, $2.4 million contract and stands to make a base salary of $600,000 for the 2016 season.
This year, Reid has appeared in seven games for the Chargers and recorded five tackles, one sack, a fumble recovery returned for a touchdown and a pass defense. Pro Football Focus has him rated as the No. 70 interior defender out of 119 qualifying players.