Milan meet Lecce this weekend.
Fonseca said: "It’s very, very important. We have to confirm that we are growing.
“The victory in the derby makes more sense if we also win against Lecce, because we need to start recovering some points.
“There are lots of motivations to win tomorrow. For me it’s a dangerous match after the derby, we have to be careful, we can’t make any mistakes. We have to win.
“I don’t want to go back to the derby, it’s over. The mental part was more important than the tactics and technique. These games are very emotional. Being in a good place, balanced, motivated and energetic is what’s important.”
He added: "A win in the derby brings confidence, it brings a different atmosphere. But I am like that, I always try to be balanced. One match can change everything, it helps the players’ confidence, too. But it’s just one match. If we don’t win tomorrow, people will say we could be relegated.
“For this reason, we have to forget about the derby, focusing on the next game, which is the most important. The atmosphere is positive, we are happy, but it’s important to understand that the derby is over, it’s irrelevant now.”