The Portuguese tactician believes his side will have a tough time breaking through Atleti's defense on Tuesday.
Diego Simeone's men have won all seven matches in La Liga so far, conceding just five goals in the process, with Fonseca comparing the team's defense to that of an Italian team.
"I consider Atletico an Italian team in a Spanish championship," the 40-year-old told a press conference. "They are very deep when they defend, but they are also very balanced. We have to be a team which manages all the moments of the game without losing that balance.
"Also, we need to have lots of patience to find the spaces which are almost non-existent in their defense."