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Goals from Federico Chiesa and Adrien Rabiot sealed a second-leg 3-2 victory for the Old Lady in Turin on Tuesday.
However, Juventus crashed out of the Champions League round of 16 as Porto progressed to the quarter-finals thanks to a 4-4 aggregate scoreline.
The 36-year-old Portuguese superstar was also slammed by fans after he turned his back in the wall during Sergio Oliveira's free-kick.
Oliveira scored his second of the match in extra-time as his strike went straight through Ronaldo's legs and beat Wojciech Szczesny to crush Juventus' Champions League dreams.
Former Juventus manager Capello blasted the five-time Ballon d'Or winner for his "unforgivable mistake" in the wall, saying there was "no excuse" for it.
The 74-year-old Italian said: "Certain goals cannot be conceded.
"Cristiano Ronaldo jumping and turning in the wall... whoever is in the wall cannot be afraid of being hit.
"It is an unforgivable mistake that has no excuse."
Del Piero, on the other hand, insisted that Ronaldo "must take some blame" after he wasted a header that Juventus "should have taken advantage of."
The 46-year-old former Juventus forward explained: "Cristiano must take some blame, of course.
"He did not have many chances but he wasted that header that we should have taken advantage of."
Juventus boss Andrea Pirlo acknowledged the costly mistake made by Ronaldo, saying it "had never happened before that they'd turn around."
The 41-year-old told Sky Sport Italia: "We choose the players to go in the wall.
"It had never happened before that they'd turn around, maybe they felt it was so far out that they didn't sense the danger.
"It was a mistake, it usually doesn't happen, the players didn't feel it was a dangerous situation and conceded a goal."
Featured Image Credit: PA
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