Mason did not practice all week and had been questionable for Sunday's game. First-round pick Todd Gurley was ruled out earlier while rehabbing from knee surgery.
Wide receiver Brian Quick, recovering from a shoulder injury, also is inactive for the Rams.
Seahawks backup cornerback Tharold Simon is out with a toe injury after being questionable. Backup linebacker Mike Morgan was ruled out earlier with a hamstring injury that kept him out of practice all week.
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