The pair picked up injuries in the 2-1 defeat to Bayern Munich before the international break.
Neither were called up for international duty as a result and they are expected to miss PSG's match at home to Le Havre on Saturday.
The French side then host Spurs in the [Champions League](/topic/champions-league) on Wednesday night and Le Parisien report there is only a "slim" chance either Dembele or Mendes will be fit for that.
That would be a significant double blow for PSG, with Dembele the current Ballon d'Or winner and Mendes the best left-back in the world over the past year.
Paris boss [Luis Enrique](/topic/luis-enrique) has also indicated that he will be now be more cautious with players returning from injury.
"When you get injured players back they are not in 100% form," he said after the defeat to Bayern.
"It is something I should handle much better given all of my experience."
The full-back suffered the injury against Bayern after a wild challenge from Luis Diaz, who was shown a straight red card.
PSG and Spurs are both still in the hunt for a top-eight finish that would earn them qualification straight through to the last 16 of the Champions League.
Spurs are currently tenth in the standings with eight points, while PSG are a point better off in fifth.
After their trip to Paris, Spurs host Slavia Prague and Borussia Dortmund before concluding their league-phase campaign away to Eintracht Frankfurt.