The hosts dominated possession in the opening 30 minutes and had several good chances to take the lead, but it was Napoli who scored first when Piotr Zielinski smashed the rebound from his own shot into the roof of the net following an incisive counter-attack.
Barca were on the front foot throughout with their brand new attacking trident formed by new signings Torres, Traore and Pierre-Emerick Aubameyang, but missed several chances to take the lead.
The two teams will meet again in Naples on Feb. 24 for the decisive second leg.
Barcelona manager Xavi Hernandez said that his side had good looks but should have scored more.
"Only thing missing was taking our chances, good pressing, good after losing the ball, we created 20-plus chances, six or seven very clear." Xavi said. "I am sad to have missed so many."
Xavi also said that despite several misses, Torres will find his proper scoring form soon enough.
"The same happened to Luis Suarez [while at Barcelona] ... he struggled to score goals in his first few months here. Ferran has a lot of personality and showed that to take the penalty," Xavi said. "[Torres] was good: how he got in the position for the chances, his pressing ... an example for the team. Delighted to have him, and no doubt the goals will go in."
Napoli manager Luciano Spalletti was not pleased with the call that allowed Barcelona to equalise the match.
"They [Barcelona] deserved a draw, but that penalty was non-existent," he said after the game.