Fremantle suffered a double injury blow in quick succession during Saturday's clash with Collingwood at the MCG, with leg injuries to defender Brandon Walker and ruckman Sean Darcy hampering the Dockers' afternoon.

Walker was stretchered off the field in the third term after awkwardly landing on his right leg from a marking contest, with his knee seen to buckle under him. Walker was in visible pain and unable to get to his feet following the contest, with the medi-cab eventually brought onto the ground to assist Walker off the field.

Fremantle's tactical sub Nathan O'Driscoll was brought into the game soon after, with the Dockers confirming he took the place of Walker on the field.

Speaking after the loss, coach Justin Longmuir said Walker will be sent for scans on what the club can only assume is a "serious" setback for the young defender.

"He's done something pretty serious," Longmuir said.

"He's sore. We will get him back home and scan him but he's going to miss a bit of footy."

The concerning setback came just minutes after Darcy limped down in his side's rooms with am ankle injury of his own. He was soon ruled out of making a return to the game.

Longmuir said Darcy's setback was a concern, with the Dockers hopeful their first-string ruckman won't be spending an extended period of time on the sidelines.

"It was a bit of an innocuous injury, the way it happened anyway," Longmuir said.

"He has gone over his ankle. We will get him back and get him scanned but hopefully it comes up alright.

Collingwood confirmed their sub change moments earlier, with midfielder Tom Mitchell replacing veteran Scott Pendlebury in a purely tactical change.

Collingwood WON BY 46 POINTS