Oshie was pushed into the boards by Carolina Hurricanes forward Warren Foegele late into the third period Thursday, crashing shoulder-first. Oshie laid on the ice for several minutes and went directly to the locker room clutching his right arm.
"It's a defenseless player quite a distance from the boards," Reirden said. "It's an extremely dangerous play. He won't be with our team for a while. He won't be playing anytime soon."
The Hurricanes won the game 2-1 to even the series at two games apiece. Game 5 is Saturday in Washington.
Reirden said the play will be reviewed by the Department of Player Safety. The Capitals took offense that there was no major penalty called on the play.
"If the guy hurt, it's a dirty play," captain Alex Ovechkin said. "It has to be not two minutes. It has to be different call."
Foegele, the 23-year-old rookie who has been a breakout offensive star for the Hurricanes during the postseason with three goals in the first four games, was apologetic afterward.
"I was trying to lift his stick, trying to give him a little nudge," Foegele said. "It was a little unfortunate play there. I wasn't trying to hurt him or anything. I hope he's OK."
Oshie missed 12 games during the regular season as he dealt with symptoms from a concussion.
The series turned testy as it shifted to Carolina for Games 3 and 4. In Game 3, Ovechkin fought -- and knocked out -- Carolina rookie Andrei Svechnikov. Svechnikov missed Game 4, as he is in concussion protocol. Carolina lost another top-six forward, Micheal Ferland, in Game 3 to injury. Ferland has no timetable to return, according to Hurricanes coach Rod Brind'Amour. In the first period of Game 3, Carolina forward Jordan Martinook awkwardly collided with the boards and had to be helped off the ice. He was later ruled out of the game with a lower body injury.
When asked about what it would mean if Oshie was out for an extended period, Brind'Amour said: "We have way more injuries than they do, so I don't worry about their team."
Brind'Amour did not see any problem with the play.
"[Oshie] crashed into the boards hard, and that's when the [official's] arm went up, because [Oshie] stayed down," Brind'Amour said. "You don't like to see that, but more than anything, he was just not ready for the hit. You see a lot of hits that are way, way worse than that. So, I think he just went in awkward."