Washington was signed two days after wide receiver Craig "Buster" Davis was placed on injured reserve with a groin injury, meaning his season is over. The Chargers also have been without wide receivers Legedu Naanee and Malcom Floyd, who have hamstring injuries.
Gates sprained the plantar fascia in his right foot during last weekend's 33-25 victory over visiting Tennessee, the San Diego Union-Tribune reported Wednesday. He also has a turf-toe injury in his left foot. The Chargers didn't list the plantar-fascia injury on Wednesday's injury report, the first of the week. A spokesman says it was an "oversight" and that it would be added on Friday's report.
Gates didn't practice again Thursday.
"In the last three weeks, he's had one day of practice," coach Norv Turner said. "I don't know if he'll be able to practice tomorrow, and I don't know if he'll be able to play on Sunday. He's getting better. The foot is there, but the toe is the issue. He's trying to deal with both things right now."
Gates wasn't in the locker room when it was open to reporters.
"He's getting better from where he was on Monday," Turner said. "That was the worst day he's had throughout this period in terms of pain. He's improved from Monday, and, obviously, Sunday was a good day for him in terms of production but a real tough day for him to get through."
Gates had five catches for 123 yards, including a 48-yarder for the go-ahead touchdown, in a 33-25 win Sunday against Tennessee that might have saved San Diego's season.
Gates was in so much pain Monday that a team trainer drove him on a cart to his car.
The Chargers waived tackle Adam Terry to make room on the roster for Washington.
Turner said the Chargers signed Washington "to protect ourselves" because rookie receiver/special-teams player Richard Goodman is injured. Washington could play Sunday and be on the punt team. "He's been very good as a gunner, so hopefully he can help us there," Turner said.
Washington is in his eighth NFL season. He has 107 career receptions for 1,327 yards and 11 touchdowns, including 34 catches for 431 yards and two touchdowns last season for the Baltimore Ravens. Washington also has made 28 tackles on special teams.
He was in camp with Philadelphia before being released and has played with Cincinnati and New England.
The Associated Press contributed to this report.