The Islanders said Hillen was being evaluated. He had skated off the ice with assistance after being hurt during the first period of New York's 7-2 loss.
Hillen was protecting the area in front of goalie Dwayne Roloson during a power play when Ovechkin wound up for a drive in between the blue line and the circles. Hillen appeared to be hit in the chin and mouth and immediately dropped to the ice with 2:31 left in the period.
"It's just the game of hockey, and it's tough when you see somebody go down like that," Capitals forward Jason Chimera said. "You don't want anybody to get injured like that, that's for sure."
Blood pooled in front of Hillen as he was tended to by medical staff. Ovechkin also checked on Hillen, who skated off despite a stretcher being wheeled onto the ice. Hillen remained down on his knees, hunched over, for several minutes before being helped to his feet.
The Islanders didn't provide any other updates on Hillen's condition immediately after the game.
"When I saw it hit him ... it was kind of a delayed reaction on Jack's part," Islanders coach Scott Gordon said.
Hillen's teammates and Ovechkin tapped their sticks on the ice in hockey applause fashion as Hillen went down the tunnel.
"I was worried for the young man," Capitals coach Bruce Boudreau said. "You could see all of our guys went over to see if he was OK, too -- Alex being the first guy. Everybody on our bench was standing up when he got up, as well as their guys.
"We hate that. When you've got kids playing hockey and you see that, you hope his face is OK."