Cofield left in the second quarter of Sunday's 17-6 loss to the Houston Texans with a high right ankle sprain.
NFL teams are allowed to designated one player on IR who can return later in the season. Under the rule, Cofield cannot practice for six weeks and cannot play for eight weeks.
The injury will end Cofield's streak of 81 consecutive regular season starts, the most among active NFL defensive tackles.
The Redskins on Tuesday also promoted safety Akeem Davis from the practice squad to the active roster.
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