Demontagnac was forced off with a hamstring injury during the first half of the defeat at Bradford on Tuesday night.
The winger joins captain Kelvin Langmead (knee) on the sidelines, but striker Clive Platt will undergo a fitness test on his back problem on Friday.
"Ishmel is making really good progress and he may still play a part this season," said Cobblers boss Aidy Boothroyd ahead of Saturday's fixture. "When it's a hamstring you have to be careful, but he may be back sooner than we first thought.
"I do have options to replace Ishmel, which is good, and we can play in a variety of different ways and with a variety of different systems.
"It may be an opportunity for Jake Robinson or I could bring Luke Guttridge or Lewis Hornby back into the team just to firm it up a little bit to make sure that we get the result that we need."
On Platt, Boothroyd added: "We really are taking it day to day with Clive because he is such a huge player for us in many different ways and to have him on the pitch or available to be on the pitch is great.
"He was touch and go for the game at Bradford but the pitch there was very hard and I wasn't sure it was the right type of pitch for him.
"He's had another week of treatment and rehab now so fingers crossed if he comes through the fitness test okay he will be available for Saturday."