"Frustrating, I don't think that even describes it," Crosby told reporters.
Still the Pittsburgh Penguins star looked for positives Monday, saying he is feeling "much, much better" than he did after being levelled by big hits in successive games Jan. 1 and Jan. 5 that triggered his 10-month absence from the game. He also said he felt worse only a few weeks before the Penguins opened training camp in mid-September.
"It's much different than previously going through that stuff," Crosby said. "I'm way better off than I was dealing with this stuff 10 months ago or whatever it was."
Still it's a setback for the NHL's biggest star, only three weeks since making a spectacular comeback.
"I just figured it was better to be cautious here and not take any chances. That's kind of where I'm at right now," Crosby said. "I'm not (feeling) bad. And I'm not happy about watching. But I've got to make sure with these sort of things that I'm careful."
Crosby had two goals and 10 assists through eight games, and his comeback appeared to be a major success until he experienced headaches following a 3-1 loss to Boston on Dec. 5.
After sitting out a pair of road games as a precaution, Crosby was expected to return to practice Monday.
But instead he sat out the training session and told reporters after that he won't practise or play until he is symptom-free. He wouldn't guess when that might be.
While Crosby absorbed several hits against the Bruins, including an elbow to the head from David Krejci, none appeared to be worrisome. He passed a concussion test on Tuesday, then went through a full practice Wednesday.
When he developed a headache shortly after that practice ended, the Penguins decided to keep him home during a brief two-game road trip in which they lost Thursday to the Flyers and beat the Islanders on Saturday.
Crosby told the team's website on Thursday that he was only being cautious, and that he planned to work out on Friday. The expectation was that he would return to practice Monday and be ready to go Tuesday.
When the headaches persisted throughout the weekend -- Crosby acknowledged having symptoms -- the Penguins weren't about to let him practise despite the absence of any test that revealed a concussion.
"The ImPACT (baseline concussion test) isn't everything," Crosby said. "You've got to listen to your body on these things, too."
Crosby missed 61 games over the second half of last season and the first quarter of this season with a concussion that resulted from punishing hits in successive games Jan. 1 and Jan. 5.
But, unlike his 20-game layoff to start this season, Crosby is experiencing headaches and other symptoms. He will miss a third consecutive game, and his 23rd this season, Tuesday against Detroit.
It was only three weeks ago Monday that Crosby lit up the Islanders -- and the NHL -- with a stunning two-goal, two-assist comeback performance in his first game since Jan. 5.
Defenceman Zbynek Michalek, who returned to practice Monday after being out with a concussion since Nov. 26, knows what the Penguins' signature star is going through.
"Now I understand," Michalek said. "It's frustrating and not an easy injury to go through. For him to have it for so long, it must have been so hard on him."
Letang, off to a big start offensively with three goals and 16 assists in 22 games, hasn't played since shaking off a hard hit from the Canadiens' Max Pacioretty to score the overtime winner in Montreal on Nov. 26. He remains out indefinitely.
What the Penguins are learning is that the recovery time for a concussion can vary widely, depending on the severity of the injury.
"There are not a lot of general comparisons in dealing with individual players," Bylsma said. "All seemed to follow different symptoms, different patterns, different recoveries, different lengths of time."
The Penguins, who went into Monday's schedule with the second-most points in the Eastern Conference, felt like they had a prime Stanley Cup contender once Crosby returned last month.
"He's been away for so long, and was so happy to be playing with us again," goalie Marc-Andre Fleury said. "It's been crazy. We've had a full team for only a couple of games."
Maryse Lassonde, a neuropsychologist and brain injury researcher at the University of Montreal, said that when a player sustains a concussion, there is a higher probability of having subsequent concussions.
"And also the brain might be more vulnerable with the repeated concussions, so that just a small hit the third time may produce symptoms that are similar to the ones you would have following one big concussion, for instance," Lassonde said from Montreal.
"I don't want to speculate, but this might be the case here with Sidney Crosby."