Riise out of Portugal clash
Riise suffered the injury during a training session on Monday at the Ullevaal Stadium.
"He was unfortunate. He seems to have suffered a concussion, and he's been taken to hospital as a precautionary measure," Norwegian team spokesman Jon Moerland said.
"I haven't been fully updated but apparently it was a fair challenge between two players and his (Riise's) head clashed with the other player's shoulder. It was just an unlucky thing."
In their opening Group H matches, Norway beat Iceland 2-1 in Reykjavik and Portugal drew 4-4 at home to Cyprus.