Gates, who had played through a torn plantar fascia in his right foot, has done very little during practices this week and is listed as doubtful on the team's injury report released Wednesday.
Gates was inactive for last Sunday's victory over the Kansas City Chiefs, and a short week of preparation doesn't appear as if it will allow Gates enough healing time for him to participate, the source said Wednesday.
Gates sustained the injury during an Oct. 31 victory over the Tennessee Titans. He missed the next two games, but he played in Weeks 12 and 13, catching 10 passes and scoring a touchdown, although clearly bothered by the injury.
Gates' 50 receptions, 782 yards and 10 touchdowns lead the Chargers, despite him missing three full games.
Wide receiver Patrick Crayton (wrist) didn't practice Wednesday and is ruled out of the game. Wide receiver Malcom Floyd (hamstring) and linebacker Larry English (foot) are doubtful after also missing practice.
Defensive end Jacques Cesaire (knee) was limited and is listed as probable for Sunday's game. Defensive tackle Antonio Garay (ankle), offensive tackle Marcus McNeill (knee) and wide receiver Legedu Naanee (hamstring) practiced and also are listed as probable. Guard Louis Vasquez (neck) is questionable after fully practicing all week.