The Giallorossi conceded twice in the final 12 minutes against last season's runners-up to kick off their campaign with a disappointing loss.
Sergio admitted it was a negative way to finish the match after playing so well for 78 minutes.
"We were doing well tactically and knew it was a tough game, so we're upset that it ended this way.
"We have to understand what it is we need to do better and at the end of the day try to get the points.
"If you lose even after a strong performance, then you have to learn from those mistakes.
"We tried our best and tactically did well to smother them, but in the final 15 minutes it all fell apart."