Deulofeu capped an impressive individual display by setting up Mario Pasalic's late winner with a cross that nutmegged defender Emil Krafth as Milan overcame red cards to Gabriel Paletta and Juraj Kucka to record the most unlikely of victories.
Milan had come into the game having lost four straight matches – three in Serie A – and would have sealed their worst league losing streak since April 1986 with another defeat.
But Vincenzo Montella's men avoided that with an astonishing team performance as they played with 10 men from the 36th minute and nine from the 59th, with Deulofeu stunned by the manner of victory.
"It was an incredible night - I have never seen a game like this, with nine against 11," on-loan Deulofeu said to Sky Italia.
"These three points feel like they are worth six. In fact, let's say they are worth 10!
"These are three points of astonishing importance and we are very happy.
"We honoured the jersey. Milan is a historic club and we must always put in performances of such determination.
"I ran to the byline and I saw Pasalic there. I don't do easy things, I try to do the difficult things and when I saw him there I had to try.
"I did think the red cards were harsh but this Milan has young players with character. Day by day, game by game, they can go far."