Mbappe's goal yesterday was prime example what you need to avoid against them. Rennes loses the ball during their attack near QSG's penalty area, one long ball on Mbappe skipping the midfield, defender doesn't engage the 1 on 1 against him and goal.
Perfect material for Xavi to prepare the team for it:
If we lose the ball and it will happen obviously, the team needs to be prepared to instantly press the player and not give them the opportunity for a precise long ball. If that also fails, either Kounde or Araujo need to engage him. If they just escort him like the Rennes defender did in this situation for fearing a foul / penalty, he will always score.