Motlop, 28, injured his hamstring playing in the SANFL last weekend.
He joined in parts of training on Thursday, but failed to make an appearance at the club's final session of the week on Saturday.
Gray, 21, is also nursing a hamstring complaint and coach Mark Williams said he wouldn't risk either player against the Roos.
"We wanted to just see how they [Motlop and Gray] pulled up and this morning. They were still a bit sore so we thought we wouldn't play them," Williams said on Saturday.
"They weren't close enough to play. We want to play fit players because North have certainly got a fit side."
Emergencies Scott Harding, Nick Salter and Matt Thomas are in the running to replace the injured pair.
Gray's body has been a concern since he arrived at Alberton with pick No.55 in the 2006 NAB AFL Draft.
The clever forward/midfielder suggested he had overcome his injury concerns by completing the entire pre-season and Williams was hopeful the latest setback was just a one-off.
"Only time will tell, but everything except the last five minutes against Collingwood down in Mt Gambier would suggest he's back on track," Williams said.
"He has a bit of stiffness, a bit of soreness, so we're not quite sure. The injury was not that dramatic that everyone knew about it. I certainly didn't know about it until the next week."
Robbie Gray is a $321,200 forward in this year's Toyota AFL Dream Team competition.
