Fremantle defender Michael Johnson has been ruled out for the next four-to-six weeks after undergoing surgery on his ankle.

Johnson suffered ligament damage in the Dockers' round 10 loss to Richmond but bravely played out the match as several of his teammates succumbed to injury.

The 24-year-old was forced to go under the knife last week after struggling to shake off the complaint.

And in a further blow to the Dockers, youngster Brock O'Brien - who has played just three games since making his debut in 2007 - will miss the rest of the season with a quad injury.