The coach admitted his side were not focused heading into the game, which was turned with an early red card to Juan Cuadrado
Juve suffered their first Serie A defeat under Pirlo in his 13th league match in charge, going down 3-0 in what was also their heaviest loss since moving to the Allianz Stadium in 2011.
Cuadrado caught Gaetano Castrovilli on the shin and was issued a red card on his 200th appearance for the Turin giants – upgraded from a yellow following a VAR check – with only 18 minutes on the clock.
Dusan Vlahovic had given Fiorentina an early lead by that point and Juve could not respond, conceding again through Alex Sandro’s own goal and Martin Caceres’ tap-in.
Juve had a couple of penalty appeals turned down in Tuesday’s match, their last of 2020, but Pirlo conceded his team simply failed to turn up.
“What happened with the red card cannot be used as an alibi,” he told Sky Sport Italia. “The result was not down to Fiorentina’s attitude but the lack of ours.
“When this is the case you face bad situations like this. In games before Christmas it can happen then you have your mind on the holidays. We were unfocused and could not recover.
“The images for the sending off seem clear to me. There could have been some decisions in our favour, but that’s not something I want to comment on – you can see for yourself.
Juve had more possession than Fiorentina (54 per cent) and more shots (13 to 10) yet ultimately saw a 13-game unbeaten run come to a shuddering halt.
Leonardo Bonucci in particular struggled – the experienced defender arguably at fault for all three of the opposition goals.
He lost possession six times in all and won only half of his 10 duels, but Pirlo stood by his skipper at full-time.
“Bad evenings like these happen to everyone,” he said. “Up until this point he has made some great performances.
“It happens to everyone. He is our captain and we still have great faith in him.”
Juve suffered a double title blow of sorts on Tuesday, the defeat to Fiorentina coming hours after Napoli won their appeal against October’s abandoned fixture between the sides.
The Bianconeri had been awarded a 3-0 forfeit win after Napoli failed to turn up due to a travel ban put in place by local authorities after Piotr Zielinski, Eljif Elmas and a member of staff had tested positive for Covid-19.
But the Italian National Olympic Committee’s Collegio di Garanzia dello Sport – the highest sports authority in the country – ruled in Napoli’s favour, meaning the game must be replayed in early 2021.
Juve are therefore three points worse off, now seven adrift of leaders Milan, and Pirlo questioned the fairness of the decision.
“We don’t mind playing the game again. I’m more disappointed for the other teams that have travelled and played without far more players who had Covid,” he said. “They showed great fairness by playing anyway.
“I won’t say if the ruling was correct or not, but it didn’t seem fair to the other teams who carried on and played in more difficult situations.”