Coach John Fox announced after practice Wednesday that the rookie from Notre Dame will start at quarterback on Sunday at Tampa Bay. Clausen will replace Matt Moore, who needs season-ending shoulder surgery.
Fox had hedged on his QB decision after Sunday's 34-3 loss to New Orleans. Clausen replaced the injured Moore, but had an interception returned for a touchdown and Fox later benched him for fellow rookie Tony Pike.
Fox says Clausen had a good practice.
Williams is not expected to play Sunday due to his foot injury, a source familiar with the situation told ESPN NFL Insider Adam Schefter. But Williams is likely to play Nov. 21 against Baltimore, the source said.
Information from The Associated Press was used in this report.