The Russian Hockey Federation also said Ovechkin, who had five goals and three assists in 13 playoff games, would not be able to play at the world championships in Europe because of the injury.
A person with direct knowledge of the situation confirmed the injury and treatment to The Associated Press.
The Capitals captain made no mention of an injury during the playoffs, which ended with a Game 7 loss to the Pittsburgh Penguins on Wednesday night. Coach Barry Trotz is expected to address all team injuries at his postseason news conference Friday.
It was not immediately clear when Ovechkin was injured or how severely. He did not miss a game but briefly left late in the first period of Game 5 of the first round after clutching his knee following a hit from Toronto's Nazem Kadri, returning for the start of the second.
Ovechkin said after that game he was not worried about his knee and "just needed a little break."
Trotz moved Ovechkin to the third line midway through the second round against Pittsburgh, slightly reducing his ice time. Trotz said he wasn't "emotionally" ready to talk about Ovechkin after the Capitals were eliminated Wednesday night.
"Emotionally right now, I don't want to answer that question," Trotz said after the 2-0 loss. "We win and lose as a team. That's probably my best answer right now. Emotionally, I don't think I want to answer that question at all."
Ovechkin has played at the world championships 12 previous times, including in 2013 when he finished the playoffs with a hairline fracture in his foot.
Kuznetsov and Orlov are restricted free agents who need NHL contracts for next season, something that often leads players to decline an invitation to play for their national team.