Brassard was rocked by a thunderous, open-ice hit from Montreal defenseman Mike Weaver in the first period, after which he was slow to get up.
He received medical attention on the Rangers' bench from the team's head athletic trainer but then headed off for the dressing room. He did not appear on the Rangers' bench to start the second period.
The team had no immediate update.
The 26-year-old Brassard, who hails from Hull, PQ, has four goals and seven points for the Rangers this postseason. He centers the team's most effective forward line that also includes Mats Zuccarello and Benoit Pouliot.