Broomhead suffered a broken leg and Scully a suspected fractured ankle in the first quarter on Saturday, with both stretchered from the ground and taken to hospital.
The brave Pies led by 11 points early in the final term but the Giants finished a gruelling contest strongly to secure a 15.5 (95) to 12.7 (79) win – just their second victory at the ground in the club’s history.
GWS midfielder Stephen Coniglio was outstanding with 27 possessions and three goals and co-captain Callan Ward led the way with 29 possessions.
Both sides suffered significant losses before the match, with Collingwood tall forward Ben Reid (hip) and Giants livewire Toby Greene (hamstring) late withdrawals.
The loss of Reid, in particular, was a huge blow for Collingwood, who were already without suspended big man Mason Cox.
Darcy Moore was swung forward to add some marking power to the Pies’ attack but he joined the casualty list with a suspected hamstring injury and didn’t play in the second half.
The heavy injury toll marred an otherwise entertaining contest.
The Giants looked set to break the game open when they stretched a five-point quarter-time lead to 17 points early in the second quarter.
But Nathan Buckley’s men did well to weather the storm, with ruckman Brodie Grundy – the only Magpie left on the ground more than 192cm tall after halftime – a key factor.
Opposed to Rory Lobb, Grundy starred with 20 disposals, 45 hitouts and two goals to go with his imposing physical presence.
The Giants’ only win at the MCG in 12 previous visits came against Melbourne in 2014.
That barren run looked set to continue when James Aish kicked the first two goals of the final term for the red-hot Pies.
But Coniglio answered with two of his own to turn the tide the visitors’ way.
Broomhead’s gruesome injury occurred when he attempted to kick a loose ball off the ground but instead smashed his left leg into a goalpost.
Minutes later, Scully was left clearly distressed when his right ankle folded underneath him in a strong tackle.
© AAP 2018
GREATER WESTERN SYDNEY 4.1 6.2 10.2 15.5 (95)
COLLINGWOOD 3.2 5.3 9.7 12.7 (79)
Goals: Greater Western Sydney: J Cameron 3 S Coniglio 3 Z Langdon 2 A Tomlinson D Lloyd H Himmelberg J Kelly L Whitfield M De Boer T Scully. Collingwood: A Treloar 2 B Grundy 2 B Crocker D Moore J Aish J Crisp J Thomas T Phillips T Varcoe W Hoskin-Elliott.
Best: Greater Western Sydney: S Coniglio C Ward D Shiel L Whitfield J Finlayson T Taranto. Collingwood: B Grundy T Phillips J Howe S Pendlebury S Sidebottom T Adams.
Umpires: Matt Stevic, Curtis Deboy, Hayden Gavine.
Official Crowd: 30,941 at MCG.