Fiorentina had lost the first leg 1-0, but when Eto'o scored at the Stadio Franchi they knew they'd need another three goals to qualify for the Final.
"We conceded that goal and lost all our confidence," said Prandelli. "It's a shame, as we could've kept up a certain tempo right to the end.
"We took a risk on an offside trap that was unnecessary and were punished for it. Now we have to give our all and end the Serie A season well.
"You can create and waste chances, but the huge mistake we made in this match was just silly. The defence cannot push up that way.
"The regret is that we were in the tie up until that point. The first half was hard fought from both teams."