TORONTO -- Having suffered at least four concussions himself, former NHL forward Keith Primeau has a good idea of what injured Pittsburgh Penguins superstar Sidney Crosby is going through.

Head injuries eventually forced Primeau from the game in 2006. He had tried for over a year to get back in the Philadelphia Flyers lineup but was eventually told that it would be best if he stopped playing.

Primeau retired a month later.

"If I knew then what I know now, the ultimate decision would have been for me to call it quits earlier," Primeau said Wednesday. "But there was no chance that I was ever going to do that. How do you convince somebody? You're playing a little bit of Russian roulette."

Crosby suffered what's believed to be the fourth concussion of his career Monday night against the Washington Capitals. His return date is uncertain.

The time may be approaching, doctors suggested Wednesday, for Crosby to take a hard look at not when -- but if -- he should return to the game.

"When there have been multiple concussions, the chance of having persisting symptoms goes up terrifically," said Dr. Charles Tator, the director of the Canadian Concussion Centre at Toronto Western Hospital. "So we're especially careful about helping people avoid further concussions.

"If he were an amateur, we would probably tell him to hang up his skates."

Crosby was out of action for almost a year after suffering a pair of head injuries in early 2011. He suffered another concussion last October but only missed two weeks of action.

"Everybody heals differently, no two concussions are the same," Primeau told The Canadian Press from his home near Philadelphia. "There's just so many variables that go into the decision making (of whether to return). When I look at somebody like Sidney who has had four concussions, I immediately go to, 'What are you waiting for?"'

Dr. Paul Echlin, a primary care sports medicine specialist in Burlington, Ont., and past chair of the London Hockey Concussion Summit, said a fourth concussion is very concerning. At that point, Echlin said, long-term effects need to be considered.

"This is a young man's life ... this is a human issue," he said. "It's not about whether the Pittsburgh Penguins advance in the playoffs or what the matchups are."

It's unclear whether Crosby will be able to return for the second-round series or at any point in the post-season.

"He's a world-class player," said former teammate Tanner Glass, a forward with the New York Rangers. "There's no way to replace him in your lineup. He's a leader too. He's a leader in that room. He's a guy everyone looks to when times get tough to lead the way. It's going to be tough to replace (him). You never like to see anyone (get) hurt like that."

Tator said that previous recovery time is an important criterion for a player to consider before making a return.

"That's what we want people to do: wait it out until you get better," he said. "But the fact that it took him a year to recover (in 2011) means that there was a significant effect on his brain. There was a residual effect."

Another big problem, Tator noted, is that many players suffer knocks to the head in their younger days and don't count it as a concussion or head injury. For high-collision sports, he'll often take the number of concussions a player has said they've had and double it.

"The most important thing is the likelihood of recovery because the likelihood of recovery goes down as the number (of concussions) goes up," Tator said. "There are lots of players who have had to hang up their skates or hang up their cleats because they didn't ever get over them."

Making things even tougher for Crosby is that his latest injury came in the heat of a playoff series against an archrival. Primeau said personal pride can sometimes get in the way of clear decision-making.

"You feel like you're against the odds or you're beating the odds," he said. "In reality you don't have the ability to look at the full picture."

Primeau's advice to any player is to listen to your body and consider long-term health. But that doesn't make the decision on a player's future in the sport any easier.

"You're taking a part of who you are and suggesting that maybe that might be the end," Primeau recalled about his decision to retire. "It's a very trying experience to say the least."

Crosby, 29, led the NHL with 44 goals this season and was recently named a finalist for the Hart Trophy. He had 11 points in eight playoff games before going down.

"If you've had repetitive traumas and the recovery time is longer, you should consider what type of activity you play," Echlin said. "Because to continue to traumatize yourself, you're eventually going to get so that you can't recover. Then your quality of life is substantially reduced."

Dr. Blaine Hoshizaki, a director of the Neurotrauma Impact Science Laboratory at the University of Ottawa, said evidence indicates that people with traumatized brain tissue will be at higher risk of developing neurological conditions as they age.

The issue for professional athletes, he added, is balancing the ability to earn a living against the risk of long-term problems.

"That's a really tough equation because for some athletes, this is their life," Hoshizaki said. "This is not a hobby. To apply a two, three or four-concussion threshold is challenging for the individual. From the medical side, having three concussions would suggest that the individual consider removing themselves from contact sport.

"But when you look at Sidney Crosby or most professional athletes, this is a different equation. Therefore to remove themselves from sport is a challenging question. That's the challenge around professional sport."