The seven-time European champions Milan are ninth in the Serie A standings, having won one of their four matches and welcome Liverpool on Tuesday.
Liverpool, who convincingly won their first three Premier League games under new coach Arne Slot without conceding, were stunned 1-0 by Nottingham Forest at Anfield on Saturday to sit fourth in the English top-flight.
"I think Liverpool is one of the strongest teams in Europe. They have a good game and strong individuals. We have to be perfect defensively, you cannot make mistakes," Fonseca told reporters.
"I see it as an opportunity to show our progress. It will lead us to do different things... but also to prove our worth.
"I am sure it will, the support of our fans is always important for us. Tomorrow it will be like that, we want to feel their love."
Milan play rivals Inter Milan in Serie A on Sunday, and the Portuguese remained tight-lipped about any potential changes in the squad.
Milan have already suffered a blow with defensive midfielder Ismael Bennacer undergoing surgery on his right calf. The 26-year-old Algeria international's recovery period is estimated to be four months.
"We have many games and I have to think about all of them. Tomorrow is definitely the most important because it's the next one, but I can't forget that on Sunday there's the derby," Fonseca said.
Milan's new signing, Alvaro Morata, is expecting a stiff test against Liverpool.
The 31-year-old Spanish striker has scored once in two appearances for Milan since joining from LaLiga club Atletico Madrid on a four-year contract in July.
"It will be a great challenge, we have so many different players and we have to fix so many mechanisms, only time will tell," Morata said.
"Ever since you arrive at a club like this you dream of playing with Liverpool in the Champions League, it could even be a semi-final, it's strange that we play right away. There is so much desire. These are the games that make sporting history."