Westhoff was expected to miss between four and six games after breaking a bone in his left foot in the round two loss to West Coast at Subiaco, but Power football operations manager Peter Rohde said further investigations suggested Westhoff's recovery time would be significantly longer.
"Westy's had a pin put in his fifth metatarsal, so the view is that it's going to be an eight-week injury," Rohde told afl.com.au
"The injury wasn't any worse (than we first thought), but when we looked into people who have had a similar injury before, it's actually taken them six weeks before they can even start training again and then they haven't played for seven to eight weeks.
"With Justin being such a tall bloke and his agility being so important to him, we're expecting an eight-week injury at least."
Westhoff's move to the long-term injury list comes is more bad news for the Power, who will also be without skipper Dom Cassisi and star veteran Peter Burgoyne when they take on reigning premier Hawthorn at the MCG on Saturday.
Cassisi (one week) and Peter Burgoyne (two weeks) both chose to accept their respective sanctions from the AFL match review panel.
Onballer Shaun Burgoyne, who copped a knock to his knee against Melbourne on Sunday, is also in doubt for the clash with the Hawks.
Westhoff's stint on the sidelines could open the door for another of the club's rookies.
Rookie-listed livewire Wade Thompson, who recently replaced the injured Jackson Trengove on the senior list, has already made his AFL debut.
But Rohde said the Power were in no hurry to replace Westhoff on the primary playing list.
"We'd like to have the flexibility there to upgrade someone from the rookie list if the opportunity arises," Rohde said.
"We'll wait and see, but at this stage we're not looking to (promote a rookie) in the immediate term."
Ex-Tiger Danny Meyer, WA product Jessie Laurie and key-position players Daniel Stewart and Matthew Martin are the Power's four other rookies.
