Williams, the No. 2 pick in last year's NBA draft, broke a bone in his left hand during practice Thursday and will be out up to eight weeks. The Hawks open the regular season Wednesday at Philadelphia.
Coach Mike Woodson said the injury occurred as Williams swiped at the ball during a routine drill, just as he was getting ready for his first season as a starter.
"It's one of those freak accidents," the coach said. "I feel for the player. Marvin put in a lot of time to get in this position. It's definitely a setback."
The Hawks were expecting to be at full strength for the first time when they played their final preseason game Friday night against the Charlotte Bobcats in Chapel Hill, N.C.
Claxton, who broke the same bone in his left hand during a Sept. 18 workout, is set to return against the Bobcats after missing the first seven preseason games. But he won't get to play with Williams.
"We are kind of snake-bit," Woodson said. "But that's part of our league. Those are things you can't control."
Williams was looking forward to returning to the city where he played his lone college season, helping the North Carolina Tar Heels win the national title in 2005. Former teammates Raymond Felton and Sean May now play for the Bobcats.
After being picked behind Andrew Bogut in the NBA draft, Williams averaged 8.5 points and 4.8 rebounds in a backup role. But his scoring average increased each month and he improved more over the summer, winning MVP honors in the Rocky Mountain Revue.
The Hawks were counting on Williams to start at power forward. Now, they'll probably have to put rookie Shelden Williams at that spot or shift Josh Smith over from small forward and move Josh Childress into the lineup.
The loss of Marvin Williams is another blow to Atlanta's rebuilding efforts. The team hasn't made the playoffs since 1999 -- the second-longest drought in the NBA -- and went 26-52 last season. Only New York and Portland won fewer games.
Claxton was the team's major acquisition during the offseason, agreeing to a four-year deal worth about $25.5 million to handle the Hawks' troublesome point guard position.
He averaged 12.3 points and 4.8 assists for the New Orleans-Oklahoma City Hornets last season and was the runner-up in voting for the league's Sixth Man Award.
"We've missed him all through camp," Woodson said. "It was good to see him back on the court running around."