Marcelo Zalayeta struck the winner for Napoli in a game which Reja is convinced they should have won by more.
"Clearly by missing so many opportunities we had a bit of apprehension at the end, but it went well anyway because we were able to close Inter down effectively," he said of the 1-0 win.
Zalayeta saw his penalty saved by Julio Cesar, one of several crucial stops by the Nerazzurri goalkeeper.
"Between January and February we were not in great shape. Today I was able to field my first choice XI after injuries and bans had prevented me from doing so and now I can look to the future with more confidence.
"I must also thank the wonderful crowd at the San Paolo, as these fans deserve to celebrate nights like this."
Inter coach Roberto Mancini admitted: "We are not going through an exceptional period of form at the moment, much of it due to the fact we lost so many players all at the same time.
"This defeat must not knock our confidence. We have a whole week to recuperate and prepare now. I was optimistic after the Roma game and I remain so. It can happen to play badly and lose a match."