Robinson (knee) was placed on the Seahawks' injured reserve Monday, Curtis Crabtree of Sports Radio 950 KJR Seattle reports.
Robinson was already ruled out for Seattle's season opener against the Broncos on Monday night due to a sprained PCL, though he will now be forced to miss at least the first four weeks of the regular season. When healthy, the 24-year-old is expected to slot into a reserve role in the Seahawks' depleted linebacker corps, so it's likely Tanner Muse could see increased opportunities after being elevated from the team's practice squad Monday, per Crabtree.