Arek Milik struck twice in the 2-1 win.
Sarri said: “In my view we let ourselves be influenced by the importance of the game and the tournament. The truth is we have a better chance of getting results if we dominate the game and take the initiative.
“These things can happen, as we had many young players in their first Champions League appearances this evening.
“Today I had a relaxed nap before the game, so I was not agitated. I am particularly happy with the result, seeing as today is my wife's birthday and I gave this to her as a gift.
“We played well in fits and starts, but I really don't like that. I got the feeling some players were a little over-awed with the occasion.
“There were a few more errors than usual, hopefully down to the emotion and tension of such an important match.
“After taking the lead, we remained in the perennial uncertainty between wanting to finish it off or control the lead. We discussed it in the locker room and I am happy that they see it the same way I do – they weren't very happy with the performance.
“The fact remains winning our first Champions League game of the season away from home is a precious result. We broke the ice, but have a lot to learn from this match."