A draw next week will see United guaranteed to finish above PSG on head-to-head record at least, meaning the pressure is on the French side to attack.
That means Fred says United need to be prepared for whatever the star-studded Ligue 1 champions throw at them.
"Man, we already know them. We know a lot about the players. I've played a lot with Neymar," Fred told Esporte Interativo.
"So before the game we try to see a lot of videos of them.
"Whether they come out on the right or the left. If they put more speed or less speed in some moves. We have to be prepared for everything there in the game.
"We know that they can decide the game at any time. So we have to be on all the time."