Neymar has been linked to former club Barcelona and LaLiga giants Real Madrid, and he was absent as PSG opened their title defence with a 3-0 win over Nimes on Sunday.
Asked if PSG can improve this season, star team-mate Mbappe told Canal+: "It depends, we will try.
"But we will not lie; we must also see what will happen with Neymar. It's not the same team without Neymar as it is with him. But we will try to do it for the fans."
Neymar has scored 34 goals in 37 Ligue 1 appearances despite both his seasons at PSG being interrupted by serious foot injuries.
PSG and Brazil team-mate Marquinhos played down the speculation as he preferred to focus on football.
"Everyone is focused on our game, and our philosophy," Marquinhos added via Canal+. "The rest, we leave it to the board and Neymar.
"They know very well what to do. We really have to think about the pitch, not the transfers."