Nerves got the better of the 28-year-old Juventus winger, who had been one of his country's best performers during the preceding 120 minutes in Fortaleza
The versatile Bianconeri winger was meant to accept the responsibility of converting the Azzurri's seventh spot kick in Fortaleza but he passed the buck to Bonucci, who blazed high over the bar, allowing Jesus Navas to decide the game - which had finished 0-0 after 120 minutes - in la Roja's favour.
"I didn't go to take a penalty because, being honest, I didn't feel it," Giaccherini is quoted as saying by Gazzetta.it.
"I spoke with Bonucci. I said, 'Leonardo, you go', because I was convinced he would have scored. It's a shame. Unfortunately, it's always a lottery.
"But it was a great performance by Italy. We played a great game. This time, we were unlucky, but we put Spain in difficulty by imposing our game on them."
Giaccherini, though, is hoping his impressive performances in the Confederations Cup will see him return to Brazil next year for the World Cup.
"It would be a life's dream," he continued. "I have done so much to get into the national team. Therefore, I will strive to hold onto this place tightly."
Giaccherini, who netted in Italy's 4-2 defeat by Brazil in the group stages, has been capped 13 times by his country.