Liverpool had to come from behind to win 3-1 at Anfield.
And with the Reds to host PSG this week in the second-leg of their Champions League round of 16 tie, Robertson says they cannot afford another slow start.
Liverpool hold a 1-0 aggregate lead and Robertson admits: "It's going to be a tough game. They showed how good they can be, an exceptional team – probably the best we have played this season.
"We have got the lead, which is important, but we can obviously play better than we did on Wednesday, that's clear and we will have to play better as we can't rely on Alisson playing the game of his life every game.
"We have to be better, we have to be better without the ball and if we do that hopefully the crowd can be behind us as we know what they can be like on Champions League nights. The aim is to get through but we have to play a lot better than we did in the first half today and the whole 90 minutes over there."
The Scot also said: "Of course, the stadium made a difference on Wednesday as their players reacted off their fans.
“More often than not on Champions League nights our players do that and we don't need to ask our fans to be at their best, it's a given and it's up to us to react off that and perform because it's not easy to get into the Champions League quarter-finals.”