If the 26-year-old Sexton does not recover in time then Ronan O'Gara will take his place at No.10 while Wallace, who was due to play in Ulster's win against the Dragons last night, will go onto the bench.
The news comes as real blow to Kidney's men who are already without skipper and talisman Brian O'Driscoll and have only won once in France since 1972.
But a statement from the IRFU refused to rule Sexton out of contention for tonight's encounter as the medical team will continue to monitor his progress.
With the game taking place at 9PM local time stories in the build-up have focused on the extreme winter conditions with temperatures expected to be as low as nine degrees below Celsius in the Stade de France.
Referee Dave Pearson gave the surface, which does not have undersoil heating, the all-clear on Friday afternoon but will conduct a further examination 90 minutes before kick-off on Saturday night.