AFP - Two quick-fire goals at the start of each half punished the Italian champions before Federico Chiesa rescued an away goal late with the second leg in Turin on March 9.
"Coming onto the pitch like that without focus is unacceptable, there was too much superficiality," former Juventus coach Fabio Capello told Sky Sport.
"In the first half (Rodrigo) Bentancur's huge mistake, in the second eight defenders were not careful against three Porto players. This cannot happen".
For Juventus it was the fourth straight away game in a Champions League last 16 which they have failed to win.
Pirlo's side had been hit by injuries with Paulo Dybala, Leonardo Bonucci, Juan Cuadrado and Arthur all out injured.
Captain Giorgio Chiellini was forced off with a muscular problem in the first half in Porto with Matthijs De Ligt ending the game limping, with Pirlo revealing that Alvaro Morato "felt faint" and had to lie down after the final whistle.
"When you concede a goal like that so early on, it knocks your confidence," said Pirlo, in his first campaign as Juventus coach.
"The players let their heads drop and Porto were able to play the game that they wanted to play.
"We shouldn’t have played like that and it’s a shame. We could have done better."
For Corriere Dello Sport it was Juventus's "worst match of the season" blasting the Turin side as "distracted and superficial".
Star striker Cristiano Ronaldo was described as "a ghost" on his return to Portugal, the newspaper continued.
"The Juventus team instinctively let itself be conditioned by Ronaldo's performance.
"An unwatchable Juve, perhaps the worst of the season saved by Chiesa's goal."
For former Juventus and Italy international Alessandro Del Piero "Juventus lacked ideas."
"This team without Arthur, Dybala, Cuadrado, Bonucci is certainly not the same side.
"But I am convinced that the return leg will be another Juve, both from an emotional and attention point of view."
Both sides are two-time European Cup winners with Juventus's last title in 1996 and Porto's in 2004.