Fremantle coach Justin Longmuir says Michael Walters was all set to be the sub against Melbourne before an unexpected late injury setback flipped those plans around.

Walters returned to form with four goals on Sunday as Fremantle crushed Melbourne by 50 points to move to fourth on the AFL ladder.

The 33-year-old had barely played over the past six weeks - injuring his hamstring early in the round 14 loss to the Bulldogs, spending the next two weeks on the sidelines, before gaining just a slither of game time as the sub over the past fortnight.

Longmuir had told Walters privately and publicly he would again be the sub against Melbourne.

But when emerging midfielder Matt Johnson was cut down by a foot injury late last week, Longmuir shuffled his cards and promoted Walters into the 22.

The move worked a treat, with Walters tallying four goals, 16 disposals and nine score involvements in arguably his best game of the season.

“I don’t really play games like that. He was always going to be the sub and then Johnno came out, so we reshuffled the team,” Longmuir said of Walters’ promotion.

“(Walters) was unfortunate to get injured (earlier this season) and someone took his spot and played well, and it’s taken a little bit of time for him to force his way back in.

“But he took his opportunities and I thought our forwards definitely got some reward for effort by the end of the day.”

Johnson will also miss Saturday night’s western derby against West Coast.

Longmuir said if they hadn’t discovered Johnson’s injury when they did, it could have led to a stress fracture.

“He’s just had a bit of foot soreness. We probably just need to give him a couple of weeks off,” Longmuir said.

Fremantle are sweating on the fitness of skipper Alex Pearce, who sustained a knock to his surgically-repaired left arm and was subbed out against the Demons.

Pearce will undergo scans to determine the extent of the damage.

If he is ruled out, Josh Draper will come straight back into the team following his impressive debut campaign.

Fremantle won the clearance battle 47-15 and the hit-outs 47-9 against an understrength Demons outfit missing star ruckman Max Gawn.

Longmuir was thrilled with his team’s clearance dominance, but he knows there’s still work to do before Fremantle can be considered the best team in the competition.

“I think our best footy is good enough to beat anyone, but I don’t think our footy is consistent enough to beat them all yet,” Longmuir said.

“So that’s what we’re striving for.”