Briochin have already beaten Ligue 1 sides Nice and Le Havre in the competition, while PSG know the risks after a tough game against fifth-tier Espaly in the round of 32 where they went behind early and needed two late goals to win 4-2.
"If you look at the other teams in Ligue 1, most of them, over half, have been knocked out," manager Luis Enrique told reporters ahead of Wednesday's away game.
"If you look at the games against Espaly and Le Mans, we had to dig in, especially against Espaly, so it is impossible to let up.
"Of course, we hope the pitch will allow us to play well. But we also need to dig in tomorrow. We often see how the underdog comes out on top. Sometimes in football, a team without attacking could win the match."
PSG are unbeaten in the league, where they hold a healthy 13-point lead over second-placed Olympique de Marseille.
With the home leg of their Champions League last 16 clash with Premier League leaders Liverpool next week and also a league game against Lille before that, the manager believes he has the squad to cope with the busy schedule.
"The most important game is the one tomorrow. And there is also an overall vision with the games coming up in the next month," Luis Enrique said.
"Of course, the plan can be changed. Because one player could play in this game, one player could play in the one after.
"We have lots of players who can play in different positions. So we can make those changes naturally. To perform at our best against Liverpool, we need to have the right amount of playing time going into it."
