Howe, 30, originally injured his right hamstring against West Coast in round five and had been closing on a return to play.

The Magpies said Howe was "troubled" during a weekend training session and after consultation with specialists, the club decided to remove scar tissue from his hamstring.

"Jeremy trained exceptionally well last Thursday and all the signs were that he would return against Sydney this week," Collingwood football boss Graham Wright said.

"But on the weekend he felt a grumble and we decided to have him re-examined today.

"Jeremy is understandably disappointed. He put so much work into recovering from a serious knee injury in 2020 and to lose a significant part of another season hurts."

A star interceptor, Howe played just four games in 2020 before a season-ending knee injury but had previously played 20 or more games in eight consecutive seasons at Melbourne and Collingwood.

In a further blow to Collingwood's defensive stocks, they'll also have to take on Sydney without key back Jordan Roughead.

The experienced defender will join Nathan Murphy in missing Saturday's game at the SCG through the mandatory 12-day concussion break period.

Both players suffered head knocks in Saturday's win over North Melbourne., with Murphy substituted out of the game.

Roughead passed his game-day concussion test but the Magpies said they had since determined he was also concussed.

"The (concussion) protocols involve a fair bit more than the match day test and with all of the factors considered our doctors determined that it is in Nathan's and Jordan's best interests to miss," Wright said.

"We spoke with the AFL, for the sake of clarity, and support the view that decisions on concussive episodes are often complex and in need of caution."

Australian Associated Press