-- Sidney Crosby never guessed he would be going through this again -- not this soon, not this season, and especially not the way he was playing.The Pittsburgh Penguins ' superstar is experiencing concussion-like symptoms again, only three weeks after returning from a 10-month layoff. For now, he's not practicing or playing, and he isn't certain when he will be back.Crosby emphasized Monday that he is feeling much better than he did when he was diagnosed with a concussion nearly a year ago -- and better than he did a few weeks before training camp began. But after being out for so long, he's not about to risk returning too soon."It's much different than previously going through that stuff," Crosby said Monday. "I'm way better off than I was dealing with this stuff 10 months ago or whatever it was."Still, Crosby, who missed the last two games, won't play again until he can engage in a full-contact practice without experiencing any symptoms."I'm not (feeling) bad," Crosby said. "And I'm not happy about watching. But I've got to make sure with these sort of things that I'm careful and (I'm) aware of making sure I'm 100 percent before coming back."Crosby passed an initial baseline concussion test last week, a day after he absorbed several hits during a 3-1 Penguins loss to Boston. While the test result was good news and he has exercised moderately the last few days, he has been bothered by concussion-related symptoms, including headaches."You have to listen to your body," he said. "Passing ImPACT (the baseline test) was encouraging, but it's not everything. … My ImPACT was much, much worse after I did it in January. This is just something I've got to be careful with."Crosby's doctors haven't given him a diagnosis, but he knows from sitting out for so long -- from the first week of January until Nov. 21 -- that this isn't the way he's supposed to feel."Either you're kind of symptomatic or you're not; I don't know the medical terms," Crosby said. "With this kind of stuff there's so many different things you could call it; it's not always clear-cut. It's not like a break or anything. I'm treating it as being symptomatic, as I've looked at those symptoms before and (have been) treated for those symptoms before. And it's the same way I'm going to treat them now."Crosby won't play Tuesday against Detroit and is listed as day-to-day."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."Penguins coach Dan Bylsma said the reoccurrence of Crosby's concussion-related problems is "obviously frustrating." Crosby went a step further and said frustrating "doesn't even describe it.""I'm not happy to be watching or dealing with this, but I have a pretty good idea (of what he is experiencing) now," Crosby said. "But I know this is not where I was before and that's encouraging."Until Crosby developed the concussion, his only injury layoff of any length occurred when was out for six weeks-plus with a high ankle sprain in 2008.Even with Crosby sidelined this season for 22 games -- and Tuesday will be No. 23 -- the Penguins are tied for second in the Eastern Conference with 38 points. Concussions have become something of an epidemic for them, with defensemen Kris Letang Zbynek Michalek and Robert Bortuzzo and forward Tyler Kennedy also sidelined by head injuries this season.Letang, one of the League's top offense-minded defensemen, hasn't played since scoring the overtime goal against Montreal on Nov. 26. His return also remains uncertain. Michalek, injured in that same game, practiced Monday for the first time since getting hurt.According to Bylsma, the Penguins are learning that no concussion is alike."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."Also missing practice Monday was center Jordan Staal , who sat out a 6-3 victory against the Islanders on Saturday with a lower-body injury.Crosby was diagnosed with a concussion after sustaining hard hits in successive games Jan. 1 against the Capitals ( David Steckel ) and Jan. 5 against the Lightning ( Victor Hedman ). For months he was bothered by headaches, a sensitivity to bright light and loud noises and dizziness. Those symptoms didn't fully vanish until just before training camp started in mid-September.After sitting out 61 games -- 41 of them last season -- the Stanley Cup winner and Olympic gold medalist made a dazzling 2-goal, 2-assist return against the Islanders on Nov. 21, and he had 2 goals and 12 points in eight games. Despite being hit multiple times, and throwing multiple hits, he had no problems in his first seven games back.Crosby believes an elbow from the Bruins' David Krejci in front of the Penguins' bench during Boston's 3-1 victory at Consol Energy Center on Dec. 5 may have been the cause of his latest problem, though he doesn't know for sure. A mid-ice collision with teammate Chris Kunitz during the third period of the same game was jarring, but was knee-to-knee."I know I got hit in the head there (by Krejci), but I felt like I was pretty good after that," Crosby said. "I didn't feel like it was anything too major. But if I had to look at one hit, yeah."Crosby didn't feel right the next day, although he skated lightly. He practiced as usual Wednesday and talked afterward to reporters about playing against the Flyers the following night. But after he developed a headache, he unexpectedly did not accompany the Penguins on a two-game weekend road trip.Now, for Crosby and the Penguins, the waiting game begins again."After talking with everyone, I just figured it was better to be cautious here and not take any chances," Crosby said. "That's kind of where I'm at right now."