Leonardo Spinazzola struck at both ends during a chaotic game, with Amadou Diawara scoring the equaliser.
“It was a difficult match, we won largely due to the excellent second half performance," Fonseca told Sky Italia.
“We had other opportunities and were solid defensively. It was very important that we get the win this evening.
“After the defeat to Milan, the team reacted well. It's important for us to understand what we need to improve and how we must react to setbacks.
“It was a great attitude and team spirit this evening. I think we won an important match for us. It's true we have made mistakes against the big clubs, there's always something missing, but we have to work on those elements."
Fonseca also said: “Diawara had been out for a long time, then Gonzalo Villar was doing well, so it was difficult to find playing time.
“Diawara came back in the Europa League and he played really well today, not just because of the goal, but his overall performance. I am glad to have this extra option if he is in good shape."