Ask anyone who's ever had one and they'll tell you there's no such thing as a mild concussion.
Head injuries are so totally unpredictable that victims and trainers can never tell for sure how long the recovery process is going to take.
Seemingly innocuous hits have kept players out for weeks, and players who've been knocked cold have returned to the lineup in days.
So there's no telling when Sheldon Souray will play again for Edmonton, but Pat Quinn isn't expecting to see him on the team's upcoming road trip to Nashville and Chicago.
"He's under observation still," said Quinn. "He won't be in for a while, I'm sure."
Souray went down Thursday after Calgary Flames captain Jarome Iginla stuck a stick into his feet while Souray was racing to a loose puck at the end boards.
The Oilers, especially the defencemen, are still upset that they lost one of their main guys to a dangerous play that the NHL has long seemed content to ignore.
"The league has to look at stuff like that," said defenceman Steve Staios.
"You shouldn't be allowed to have a stick on anybody going into the end boards, even on his hands because that can influence the direction and the way that he's going.
"There's no need for it. If you can call hooking when you have one hand on your stick in the neutral zone, why isn't it a penalty going into the end boards?
"Very simple question and it need to be asked directly."
Staios is just back from his own concussion, suffered in the second game of the season against Dallas.
"They ask you a bunch of questions and go through a few tests, they have a whole protocol that they follow," he said.
"They test you after the game you got hit in and see if there's any improvement."