The team announced the setback.
Injuries have been a defining trait for Allen. Last year marked the first time he played a full 16-game schedule since the Chargers drafted him in 2013.
Allen's 16 games played in 2017 surpassed his total from the previous two seasons combined (nine). That health translated into an excellent performance for the 26-year-old, who set career highs with 102 receptions and 1,393 receiving yards.
This season, Allen has tallied 1,074 yards on 88 receptions. His best game was in Week 13 against the Pittsburgh Steelers, as he had 148 yards on 14 receptions in the Chargers' 33-30 comeback win.
Last year's first-round pick, Mike Williams, and Tyrell Williams will likely see their roles grow until Allen returns.
The Chargers have looked like one of the AFC's best teams. Losing Allen for any length of time is a serious pendulum swing as the franchise attempts to keep up with the Chiefs and other contenders in the conference.