Williams apparently suffered the injury in the Chargers' Week 1 victory over the Indianapolis Colts.
Head coach Anthony Lynn told reporters Wednesday he's "absolutely" concerned about the third-year wideout's knee and added rookie Anthony Johnson to the team's practice squad "just in case," according to Gilbert Manzano of the Orange County Register.
The degree and location of the reported soreness have yet to be disclosed. It's possible Williams misses Los Angeles' Week 2 matchup with the Detroit Lions, Anderson adds, though a definitive status hasn't been determined.
Williams did not practice Wednesday.
The 24-year-old is coming off a breakout 2018 season that saw him catch 43 passes for 664 yards and 10 touchdowns. Travis Benjamin, Dontrelle Inman, and Geremy Davis will likely see their snaps increased should Williams miss time.