Lynch underwent sports hernia surgery Wednesday and will be out three to four weeks, the Seattle Times reports.
Coach Pete Carroll didn't rule out that Lynch could be done for the season, but he said doctors were "very optimistic" that they fixed the issue and that Lynch can return quickly. Thomas Rawls will start while Lynch is out.
Richardson (hamstring) is expected to return to practice Wednesday, John Boyle of the Seahawks' official site reports. "He's got a great shot," head coach Pete Carroll said Monday. "We're counting on him coming back and practicing, at least in limited fashion Wednesday just to prove it, but we think he might be full-go on Wednesday."
Activated from the PUP list on Nov. 14, Richardson made his season debut in Week 10 but suffered a hamstring injury during that contest. He's subsequently missed the past two games, but an appearance in drills Wednesday would set the foundation for a return to action.