Port Adelaide will be without key forward Todd Marshall for Round 21, with the 23-year-old placed in AFL health and safety protocols.

The Power confirmed on Thursday that their sharpshooter won't face Richmond on Saturday night due to the isolation period, which he entered earlier this week.

The news is a major blow for Port Adelaide's chances of taming the Tigers, who are still pressing for a finals spot with three rounds left in the home and away season.

Marshall has booted 39 goals in 2022, having played in all 19 games so far this season.

The Power will welcome back defender Riley Bonner from COVID protocols, while utility Dan Houston will be free for selection after undergoing concussion protocols.

The loss of Marshall could see Ken Hinkley recall either Sam Hayes or Brynn Teakle for his ruck planning, freeing up Jeremy Finlayson to return as a full-time forward target.

CAIRNS, AUSTRALIA - APRIL 30: Tom Campbell of the Saints contests the ball with Sam Hayes of the Power during the round seven AFL match between the St Kilda Saints and the Port Adelaide Power at Cazalys Stadium on April 30, 2022 in Cairns, Australia. (Photo by Ian Hitchcock/Getty Images)

Marshall will exit health and safety protocols next week, lining meaning he'll return against Essendon next Sunday.

Port Adelaide will confirm their 22-man side to face Richmond at 6:20pm (AEDT) on Thursday night.