Alvaro Morata, Theo Hernandez and Christian Pulisic sealed the victory with three quick-fire goals just before the break, scoring all of them inside five minutes.
"Today it was too important to win, after the derby there is always a state of euphoria," Fonseca told DAZN.
"The players played a serious match, it wasn't all perfect but we had good moments and chances, and we didn't concede goals which is always important."
Milan had secured a hard-fought 2-1 win over Inter Milan last weekend and have now won three consecutive league games for the first time since the period between March and April.
"This team has to learn to play this type of game, our positional play has to be different," Fonseca added.
"During the break we talked about many details that need to be improved. We have to wait for the right moment, understand where the space is. It's important to improve this, the team has to learn to play like this... In the second half we did better."
Milan repeated with Tammy Abraham and Morata up front, with the latter scoring his second goal for the team.
"It was both tactical (choice) and for their aggressiveness," Fonseca said.
"Morata's characteristics allow me to have him play this role of attacking midfielder, between the lines. With a forward like (Erling) Haaland you couldn't do it.
"Then I really like the energy they both bring, it's contagious for the team. Now I also see (Rafael) Leao working harder, also because he follows their example."
Morata, who was a doubt ahead of the game with a minor injury, was substituted after 55 minutes by Ruben Loftus-Cheek.
"I'm fine, I only left the pitch as a precaution," said the Spanish striker.
"It's wonderful to celebrate with my children, they give me the strength to go to work every day. It was nice to be able to hug them, I'm very happy."
Milan travel to German champions Bayer Leverkusen in the Champions League on Tuesday after their opening 3-1 home defeat by Liverpool earlier this month.
"We immediately found the two teams that play better at the moment in Europe," Fonseca said.
"First Liverpool, now Leverkusen, who are the same as last year (or) maybe even better. We know it will be difficult."
