Casillas (neck) is listed as out for Sunday's game against the Rams, Dan Duggan of NJ.com reports.
Casillas wasn't able to participate in practice at full speed this week, so this doesn't come as a complete surprise. Since neck injuries have tricky recovery periods, the veteran linebacker will likely need a full practice before he's truly ready to get back on the field. Keenan Robinson will continue seeing a hefty workload in Casillas' absence.