Schaub was placed on the inactive list Sunday. Bryn Renner will be Clausen's backup.
Clausen was signed off waivers by the Ravens on Nov. 24, two days after starter Joe Flacco was lost for the season with a knee injury.
Schaub started the past two games for Baltimore, but did not practice this week because of a chest injury.
This will be Clausen's second start against Seattle this season. He went 9 for 17 for 63 yards for the Bears in a 26-0 defeat on Sept. 27.
Clausen will be the sixth quarterback since 1950 to start against the same team with two different clubs, according to STATS. The last was Kyle Orton in 2011.
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