The Italy international scored two of the goals against Gus Poyet's team as the Saints ran riot at St Mary's, and said that he "felt sorry" for their opponents.
"Sometimes we felt sorry for them because it is not nice for them to lose so much, but it is our job," he told Sky Sports News.
"It is an amazing day for everybody. We knew the second half we had to do better, because we were not happy how we played, but it was an amazing match
"We were happy and tried to do our best and we did it today."