The Clemson product wore his college No. 7 during offseason work. However, according to league rules, wide receivers can only wear numbers 10-19 or 80-89 during the regular season.
Williams initially said he did not want to wear a number in the eighties, but changed his mind because he will wear No. 81 during his rookie season.
Other number changes for the Chargers include rookie running back Austin Ekeler moving to No. 30; new offensive lineman Michael Schofield will wear No. 78; cornerback Trevor Williams has switched to No. 24; new cornerback Jeff Richards will wear No. 27 and new linebacker Hayes Pullard is wearing No. 50.
Williams, still recovering from a lower back disc herniation that has kept him out since May, was limited to work on a side field, along with tight end Sean McGrath (knee) and defensive end Jeremiah Attaochu (hamstring).
Linebacker Denzel Perryman, placed on the injured reserve this week after recent ankle surgery, watched from the sideline.
All other Chargers worked with their position groups, including players nursing injuries over the past couple weeks like cornerback Casey Hayward (hamstring), right tackle Joe Barksdale (foot), receiver Dontrelle Inman (sports hernia) and receiver Tyrell Williams (groin).
The Chargers are not required to publish an injury report until Thursday.