Jackson hurt his left hand on a 7-yard run on Buffalo's second play from scrimmage, and coach Chan Gailey said his team's leading rusher last season will miss the rest of the exhibition games.
"We hope to get him back" for the start of the regular season, Gailey said.
Six plays later, Lynch injured his ankle on a 20-yard run that was wiped out by a holding penalty.
"Don't know the extent of it," Gailey said, "but we're hoping it's a lot more short-term than it is long-term."
Jackson gained 1,062 yards on 237 carries in the 2009 season, and Lynch was second on the team with 450 yards on 120 runs.
"When we lost those two guys, we lost an awful lot of stuff we were trying to do in this ballgame," Gailey said. "Those of you that watched practice saw a lot of the two-tailback stuff, and we were trying to use that quite a bit tonight, and that kind of went out the gate when those two guys went down."
Without the team's top two runners, first-round draft pick C.J. Spiller gained 16 yards on four carries against Washington, a week after arriving at training camp to sign a contract.
Chad Simpson, who was with the Indianapolis Colts the past two seasons, led Buffalo by rushing seven times for 67 yards. Joique Bell, an undrafted rookie free agent from Wayne State, carried five times for 52 yards, including a 28-yard touchdown run.
"If coach needs me to step up, I'll step up," Bell said. "I'm here to do a job."