Klopp was sent to the stands for his protests after Gonzalo Higuain's first-half opener for the hosts, with the German angry that Neven Subotic had not been allowed back onto the pitch after injury, leaving his team short in defence as the Argentine striker glanced home Juan Camilo Zuniga's cross.
Things then went from bad to worse for last season's beaten UEFA Champions League finalists, as they were reduced to 10 men when goalkeeper Roman Weidenfeller was dismissed for handling outside the area just before half-time.
But even though Lorenzo Insigne went on to double Napoli's lead with a superb free-kick after the interval, Dortmund fought back well and got themselves a lifeline through Zuniga's 87th-minute own goal.
They could not force a late equaliser, but Klopp believes the Bundesliga leaders' attitude in testing circumstances will hold them in good stead for the rest of their Group F fixtures in the Champions League.
"We definitely did not show our best, but nothing is lost yet - we knew ahead of the game that Napoli were a very strong side," he said.
"The second-half performance was better and we managed to score a goal and hit the crossbar despite having a man less on the pitch. So not everything was bad."
Dortmund midfielder Nuri Sahin echoed his manager's thoughts, saying that while Napoli were the better side, the Bundesliga outfit showed their spirit by battling hard to get back into the game despite being down to 10 men.
"All things considered, it was a deserved defeat. Napoli passed the ball well and we tried everything we could," the Turkey international stated.
"I don’t think the players can be blamed, Napoli simply are a quality side and when you are down one man, it is very difficult."
