Teams: Derby County, Burton Albion
He now faces a number of weeks out and the news is a blow for the Rams, who thought the 25-year-old Scot would be back in action much sooner.
Two scans last week showed no signs of ligament or bone damage but the injury was not responding to treatment as quickly as expected.
Malcolm (pictured left) has now seen a specialist and a piece of floating bone appears to be the cause of the trouble.
"It's a case of trying to move the bone with treatment or remove it with surgery," said manager Billy Davies. "We will know more in the next few days but Bob could be out for weeks."
Malcolm sat out Saturday's 1-0 home defeat by Birmingham City, the third game he has missed since being injured in training.
Another training ground injury ruled skipper Matt Oakley out of the Birmingham match. The 29-year-old picked up a slight knee injury on Friday.
"We don't think it's too serious and I would expect Matt to be available for Saturday's game against Cardiff City," said Davies.
Defender Dean Leacock should also be in the squad to face leaders Cardiff at Ninian Park.
He missed the Birmingham defeat on medical advice after suffering a bout of concussion during last Tuesday's victory over Queens Park Rangers.
"The doctor was concerned about throwing Dean back in so soon," said Davies.
"Dean did some jogging in training late last week but no heading."
Davies says striker Paul Peschisolido faces another three or four weeks out with his thigh injury.
Bad news about Malcolm, though with Seth returning to form it's not so bad, I hope Oakleys fit for saturday, he's been a bit of colour the last few games, but Bolder and Seth are a bit too much the same to play together for me.
Originally Posted by Nico
I dont belive in jinxing mate, I do belive in playing at home with a 2-0 advantage against a team which I cant see scoring 2 goals!
Of course, if we lose, I'll never admit it was me who wrote this.
Forest 2-5 Yeovil aet
Brian Clough - Legend!