The Frenchman has admitted that it was his mistake that led to the Catalans' first goal of the evening, and added that it was very hard for his team to deal with the pressure
The Frenchman lost possession to Lionel Messi inside his own half, which eventually led to the spot kick which was converted by the Argentine in the 11th minute to help the Catalans on their way to a 3-1 win and a spot in the semi-finals.
Mexes added that it was a difficult evening for the AC Milan defence due to Barcelona's constant pressing, and felt the second penalty eventually cost the Rossoneri.
"I'm sorry because my mistake led to their first goal," Mexes said to reporters.
"It's hard to play against them because they were always pressing the defenders, so it was very difficult to start things from the back when we had the ball.
"However, we managed to equalise and we started to believe we could hit them again on the counterattack – until the other penalty."
AC Milan resume Serie A action on Saturday when they lock horns with Fiorentina.