When the 2015 season ended, Rivers was throwing to practice squad receivers and their backups.
Benjamin, 26, had a breakout season with the Cleveland Browns. He caught a career-high 68 passes for 966 yards and five touchdowns.
Keenan Allen is the team's top receiver but returning from injury the Chargers are optimistic he regains the form he showed in 2015, when he was on pace to set an all-time record for receptions.
Malcom Floyd's retirement opens a spot in the starting lineup in San Diego. General manager Tom Telesco said at the 2016 NFL Scouting Combine that the Stevie Johnson was a pleasant surprise because of his ability to easily separate from coverage. Using Johnson and Benjamin in a three-wide set with Allen could be a production design for returning offensive coordinator Ken Whisenhunt.