Rafael Benitez's side had struggled to test Samir Handanovic in the Inter goal, but after a defensive mix-up they took the lead with 11 minutes to play, Jose Callejon firing into the back of the net.
Fredy Guarin dragged Inter level with a close-range finish, but Napoli were ahead as the game neared stoppage time.
Lopez found room on the edge of the penalty area and delivered a perfect ball to Callejon at the back post, the 27-year-old side-footing his volley beyond Handanovic into the bottom corner.
However, Napoli were unable to see the game out as Hernanes snatched a point for the hosts in stoppage time, much to the frustration of Lopez.
"We just lacked concentration in the finale, which caused defensive errors," he told Sky Sport Italia.
"Now we have to turn our minds to the next games.
"I am very happy to have helped the team. Now I need to work hard and progress."
The draw sees Napoli miss the chance to break into the top six, with Lazio climbing above them after a 2-0 win over Fiorentina.
