Demontagnac, 24, pulled up in pain after 32 minutes at Valley Parade on Saturday while attempting to sprint for the ball.
Cobblers boss Aidy Boothroyd does not expect the player to feature again during the last four matches of the campaign as the club chase automatic promotion from npower League Two.
"I think that will be his season over," said Boothroyd following the 1-0 defeat to the Bantams.
"We don't really know at this stage but it will probably be a fairly long one.
"It's that stage of the season where you get these things. He's done well for us recently and it will be a blow.
"But it will give Jake Robinson the chance to have a run in the side and show us what he can do."
Demontagnac joins club captain Kelvin Langmead (knee), Clive Platt (calf) and Alex Nicholls (broken leg) on the Northampton injured list.
- Sports & Recreation
- Aidy Boothroyd