FREMANTLE has suffered a blow to its premiership ambitions with defender Michael Johnson ruled out of the Dockers' finals campaign due to a back injury.

Johnson consulted a specialist on Monday and has been advised to undergo exploratory surgery.

“Michael played out Saturday’s game under some discomfort,” Fremantle's general manager of football operations Chris Bond said.

“We were hopeful that it could be treated and he would be able to play during our finals’ campaign but the medical advice we now have is that there are no other treatment options available and an operation was considered the only appropriate course of action.

“Michael will have the procedure this week.”

Johnson has been a key component of the Dockers' backline this season, playing 18 games and averaging nearly 20 disposals per game.

Fremantle coach Ross Lyon did not hide his disappointment at losing last year's All Australian defender.
"He'll be sorely missed," Lyon told 6PR on Monday night.
"But in saying that it's one out one in and it's a really exciting opportunity for whoever comes in."
Lyon declared Alex Silvagni a certainty to play after he was a late withdrawal from last Saturday's win over Port Adelaide due to match-ups.
Luke McPharlin is still battling to recover from a calf problem that has kept him out for the past three matches. Lyon said the veteran was highly unlikely but did not rule him out.
The Dockers also have the option of throwing forwards Chris Mayne or Matt Taberner into defence as both men have filled roles in the back half at different stages this year.
"That's the beauty of our flexibility," Lyon said.
"(Mayne has) played down back and (Matt) Taberner has been down there so he could be a likely one down there."