“I have to make sure with these sort of things that I’m careful,” he said. “You have to listen to your body.”

Photo

Crosby has learned from recent history. Several former N.H.L. stars have had their careers shortened after sustaining multiple concussions.

Last Wednesday, Crosby met with Dr. Michael Collins and passed an Impact test, the standard concussion assessment test. Penguins Coach Dan Bylsma said Collins did not see indications of a concussion.

But Crosby had concussionlike symptoms — including headaches — after light workouts last weekend. He said he was not sure whether the symptoms were a result of a new concussion.

Dr. Robin Green, senior scientist at Toronto Rehabilitation Institute and an expert in traumatic brain injury, said it was not unusual for Crosby to still feel symptoms after passing an Impact test.

“All of our clinical tests have very limited sensitivity,” she said. “People can look perfectly intact, including on the Impact test he took. Part of the big problem of figuring out when it’s safe for an athlete to return to play is our tests lack sufficient sensitivity to tell us that a person is completely recovered.”

Green added that it was unclear “whether the brain ever fully resets” after a concussion. “One study showed that a year after a single concussion, a given individual shows definitive effects using a particular type of neuroimaging,” she said. “That was after just one concussion, and Crosby may have had more than one.”

Advertisement Continue reading the main story

Crosby played eight games and had 12 points in his comeback, including two goals in his first game back Nov. 21 against the Islanders — the only ones he had scored.

Photo

Pat LaFontaine, the former Islanders, Sabres and Rangers center, retired at 33 after a series of concussions. Eric Lindros played fewer than 40 games in each of his last three seasons before retiring at 34 with postconcussion syndrome. Keith Primeau retired at 34 in 2005 after sustaining four concussions.

Rangers defenseman Marc Staal sustained a concussion last February, but missed only four games before returning to play the rest of the season — except for a two-game break in March when he had concussion-related symptoms. Staal has not resumed practice with contact, and his availability this season remains in question.

Marc Savard, a center with the Boston Bruins, sustained a concussion when he was blindsided by a hit from Pittsburgh’s Matt Cooke on March 7, 2010. Cooke’s check, considered legal at the time, led to a rule change penalizing hits to the head. Savard returned for 25 games the next season, sustained another concussion Jan. 22 and has not played since.

Newsletter Sign Up Continue reading the main story Please verify you're not a robot by clicking the box. Invalid email address. Please re-enter. You must select a newsletter to subscribe to. Sign Up You will receive emails containing news content , updates and promotions from The New York Times. You may opt-out at any time. You agree to receive occasional updates and special offers for The New York Times's products and services. Thank you for subscribing. An error has occurred. Please try again later. View all New York Times newsletters.

On Monday, the Philadelphia Flyers announced that Claude Giroux, the leading point scorer in the N.H.L. with 39, would miss at least Tuesday’s game against Washington after sustaining a head injury Saturday.

Crosby said he felt better than he did when he was recovering this summer. Crosby practiced for a month before he was cleared to return.

“I’m way better off than I was dealing with this stuff 10 months ago or whenever it was,” he said.

But for now, the comeback of the best player in hockey has been derailed.