The Chile international gifted the Partenopei a 1-0 victory at the Stade de Geneve when he allowed a long-range strike from Blerim Dzemaili to slip through his hands and into the net.
However, Rakitic has leapt to his team-mate's defence, arguing that his fellow new signing should not be castigated for the mistake.
"These are things that can always happen and it does not change the confidence we all have in him," the Croatia international told reporters after the game. "Therefore, there is no need to kill him. He had a good match."
Rakitic failed to shine in his first start for Barca following his summer switch from Sevilla for an estimated €20 million (£15.9m) transfer but is pleased with how he is settling in.
"I feel better every day and I understand more with my team-mates,” the playmaker remarked. "I'm happy and want to keep improving. We always want to win. I believe that we have created a lot of chances and it is good to increase the confidence.”
Barca winger Pedro is also of the opinion that the Catalans are progressing well under new coach Luis Enrique.
"The team is good, gathering pace and confidence," the Spain international stated. "We've improved some tactical aspects.
"The result is a shame but we're feeling good. There is plenty of time to correct what needs to be corrected; the results sure will come."