NORTH Melbourne has been dealt a blow ahead of Saturday's crunch game against Hawthorn with key big men Ben Brown and Scott Thompson ruled out.

Brown (knee soreness) and Thompson (adductor) were both cleared of serious injury, but have failed to prove their fitness for the Hawks clash.

Brad McKenzie and Ryan Clarke, who both made way for returning pair Lachie Hansen and Robin Nahas on Thursday, are back into the side.

Young forward Ben McKay – who confirmed a two-year contract extension on Friday – and defender Joel Tippett have been added to the squad as emergencies.

Brown, who has played every game this season, suffered a knee injury in the second quarter of last Saturday night's clash against the Western Bulldogs.

The injury comes at the worst possible time for the Kangaroos, with tall forwards Jarrad Waite (hip) and Mason Wood (knee) also unavailable.

The 200cm Brown is North Melbourne's leading goalkicker, with 36 this season as the club's main target in attack.

Thompson has also played every game this year, but went off late in the clash against the Bulldogs with a groin problem.

McKay, who is yet to make his AFL debut, had his contract extended until the end of 2019 on Friday.

The young key forward has enjoyed an impressive VFL season, with his defensive pressure a standout trait.

Kangaroos list manager Cam Joyce said the 18-year-old had earned an extension, despite not yet featuring at AFL level.

"Ben has shown plenty of promise and now looks right at home in Werribee’s VFL side," Joyce said.

"We believe he will play a big part in the future of this footy club and we’re looking forward to watching him improve and grow both on and off the field."