Dortmund slipped to a 2-1 loss at the Stadio San Paolo in their opening UEFA Champions League Group F clash on Wednesday, goals from Gonzalo Higuain and Lorenzo Insigne proving enough for the hosts, despite a late own-goal from Juan Camilo Zuniga.
The defeat proved costly in more ways than one, with goalkeeper Roman Weidenfeller sent off and Hummels forced off shortly before the half-time interval with a back problem.
However, Klopp believes the injury is not serious and expects the Germany international to make a speedy return.
"He has received a blow in the back, on the sciatic nerve," Klopp said.
"He still has a lot of pain, but I assume, however, that it will be better relatively quickly."
Klopp was also shown a red card in the wake of Napoli's opener. The 46-year-old was sent to the stands for confronting the fourth official after the referee had refused to stop play for an injury to defender Neven Subotic in the lead up to the goal.
The former Mainz manager was quick to take the blame for the defeat, accepting that his conduct had been detrimental to his side's fortunes.
"I apologised straight after the game to the fourth official, the referee and my team as my emotions turned a game we didn't play well in into a hectic one," he said.
"The second half with a man down was okay. On good days, when the coach keeps his cool, we might get a result. Our start wasn't best, mine included.
"Had we played to our level we could have won the game. In this group everything is still possible. We will not throw anything away. We lost an away game, nothing else."
