Lightning general manager Steve Yzerman announced the injury during the first intermission of Saturday's game between Tampa Bay and Carolina.
"You have to be very careful," Hedman said after the Lightning's 5-2 win over the Hurricanes. "I feel a little bit better than when it happened, so that's progress. There's no timetable for my return."
Hedman was hurt in Tuesday night's game against Philadelphia by what Yzerman called a clean check.
The defenseman has averaged almost 22½ minutes a game this season, He had two goals and four assists.
"I'm trying to get better each day and see how it goes," Hedman said. "See how the symptoms work out. Have to make sure I'm symptom-free when I get back on the ice, and right now I'm not. So that's why I'm not playing."
Hedman has not resumed working out.