The former Brighton fullback struck the only goal of the night against the Serie A leaders.
He said afterwards: "The truth is, it was a wonderful night. I'm happy and pleased to have helped the team, to have brought great joy to all the fans tonight."
On coach Max Allegri, Estupinan also told Telelombardia: "The coach has always given me confidence: this is very important. I've worked hard for this moment. I think we have a very strong team, with great players and great people.
"The important thing is to stay ready, so when it's your turn, you can bring the best version of yourself to the pitch."
The win took Milan into second-place on the table, seven points behind Inter.
Estupinan added: "I think nothing is decided until the final whistle. We'll work every game, step by step, to bring joy to the fans. That's what we're doing. We'll fight every game.