The 41-year-old had persisted with a back three at United, employing the system in 45 of his 47 Premier League matches in charge of the club, of which he won just 15.
He also guided United to their worst-ever finish of 15th in the 2024-25 season.
Amorim's rigidity became a point of frustration for fans and former United players, and while he briefly shifted to a back four towards the end of his stint, the damage was already done and he was sacked by United in January.
Sunday's victory ended a three-match winless run for Milan and Amorim told DAZN he had tried to learn for his experience at United.
"We had difficulties in the last games, because the structure was really blocked in the positions, but the players had a little more freedom, and they played better with a little more freedom," he said.
"What I felt in the last games is the same that I felt at Manchester United, and I cannot make the same mistake.
"Sometimes we don't have the perfect characteristics to play in that way, and if we don't have it, we have to adapt.
"I think today it worked, in another game we can return to the build-up with three, the important thing is to have different ways of playing."
Milan next face Sassuolo on October 11.