"We made a huge mistake, as the game was heading towards a draw at that stage," Mandorlini told Sky Sport Italia.
"That's how it went, these things happen in football. The referee made the right decision, as it was a clear penalty.
"In the second half we came out of our shell and created four or five great chances, but got the final pass wrong. It was a good performance apart from the opening 15 minutes, as we knew where Milan wanted to move the ball."
Verona are currently sixth in the Serie A table and are level on 32 points with fifth-placed Inter Milan.