Fremantle defender Michael Johnson is in doubt for Friday night’s AFL preliminary final against Hawthorn because of a back injury, according to reports.

Johnson missed last year’s finals series because of a serious back complaint, while he was sidelined for nine weeks earlier this season after tearing his hamstring.

Reports have surfaced that Johnson received ongoing medical treatment this week for a flared disc injury.

Should Johnson be ruled out of the preliminary final, it would be a massive blow to the Dockers – also without All-Australian defender Luke McPharlin (calf).

Back-up defender Alex Silvagni has been named as an emergency, and is likely to be called into the side if Johnson fails to play.

Silvagni hasn’t played since copping a four-match suspension for elbowing Jamie Cripps in round 20 in a spiteful western derby.

Johnson appeared somewhat hampered during Fremantle’s nine-point win over Sydney a fortnight ago in the qualifying final.

But coach Ross Lyon dismissed any injury fears when asked earlier this week about the 204-game veteran.

“He was very crook when we played Sydney,” Lyon said of the 2013 All-Australian.

“He looked off. It certainly wasn’t his hamstring or back – he just came off a heavy flu.

“He was sore. I said, ‘Mate, do we need to sub you out?’.

“But he was fine.

“He’s in really good spirits. He trained fully (on Monday).”

The Hawks are without star forward Jack Gunston, who is still nursing an ankle injury.