Fonseca insists fatigue was a factor for the limp display.
He said, “It is a difficult moment, but I think we had a good first half after the defeat in Manchester. The lads tried hard, then as the half was coming to an end, we conceded that goal that is becoming something of a habit for our team.
“We didn't have the energy to react after the break, but it was a good first half.
“The loss in Manchester made its mark on us and it is difficult to get past it."
Fonseca added: “Up until March, we were challenging in both Serie A and the Europa League. I don't think we gave up on Serie A, it's just that we came here missing 10 players. We've had injury issues since March and there are players who are difficult to replace.
“That was also a time when we had a lot of games in a short space of time, so missing big players made the difference."