Pittsburgh Penguins superstar Sidney Crosby's concussion symptoms have returned, and he may not be ready for the start of the NHL season, sources have told CTV Atlantic.

The Cole Harbour, N.S., native has not played a game since January 5, 2011, after he suffered his second hit to the head in less than a week.

Sources say Crosby's concussion symptoms returned last week and he cancelled all of his scheduled on-ice workouts at a Halifax-area rink.

Penguins players are expected to appear at training camp in mid-September and Crosby is considered doubtful to be one of them, seven sources close to Crosby told CTV Atlantic.

However, Crosby's agent has refuted the report.

"Sidney hasn't been shut down by anyone. He has simply adjusted his summer program accordingly to the different needs for the appropriate recovery," Crosby's agent Pat Brisson told ESPN Monday. "Training camp is in three weeks from now. It is too premature to speculate all kinds of deliberate information at his point. Sidney will address the media at the appropriate time in order to give everyone an update. As far as I know training camp hasn't started yet."

Pittsburgh coach Dan Bylsma told NHL.com last week that he did not ask Crosby about his health this off-season.

In addition, Penguins general manager Ray Shero did not deny earlier reports that Crosby might not be able to start the season.