The 51-year-old, who has been with Milan since June, has already faced pressure due to a lacklustre start, with two draws and a loss before the comfortable home victory over Venezia, with all the goals coming inside the first half an hour.
"Our season already started four rounds ago, we dropped points and need to recover lost ground, but it was important to win and to win like this. It gives the players confidence to keep growing," Fonseca told Sky Sports Italia.
While Saturday's victory offered some relief for the Portuguese coach, the real test awaits when Milan host Liverpool in their Champions League opener on Tuesday followed by a derby against Inter next weekend.
"I have to be honest, I've already started thinking about both games against Liverpool and Inter. I realise the importance of the derby, Liverpool will be very difficult, but I cannot deny I have already started studying Inter too," Fonseca said.
"I know what it means to the fans, we are working to have good performances in both matches."
Fonseca denied that recent protests from fans, including banners placed in the stands just before kick-off warning that things needed to turn around quickly, threw him off balance.
"I think as a coach that I have to deal with these situations with balance, stay focused on my work and ignore everything around us," he said.
"...Milan is a club that always has the pressure to win. If we coaches don’t want this pressure, then we shouldn’t be coaches."