The Reds were hoping to secure a second win of their European campaign following victory against Paris Saint-German a fortnight ago.
Instead, the Reds barely turned up in Naples - failing to muster a shot on target whilst looking disjointed and sloppy in possession.
They were punished in the final minute when Lorenzo Insigne converted from Jose Callejon's low cross from the left to take all three points and top the Group C table.
And Wijnaldum suggested Wednesday night's result at the Stadio San Paolo served as a ‘reality check' for him and his teammates and they must learn their lessons.
“You're always disappointed when you lose a game. We didn't play well and they deserved to win. They had a good chance before the goal,” he told Liverpoolfc.com.
“They were calmer than us on the ball, they defended well and stopped us creating chances. It was not a good day for us. They deserved to win.
“It's a good game to learn from. That's what we're going to do – we're going to analyse the game and see what we can do better.
“We know what we can bring, we still have confidence. But this was a reality check on how things must not go. It's a good lesson for us to get better. We have to take it as a lesson and then we have to carry on.”
Liverpool had just 43% possession on the night as they looked toothless with the ball, misplacing several passes and running into trouble far too many times.
“We wanted to rush things and play football, instead of keeping the ball and waiting for the moments,” the Dutchman added.
“They did well; they were calm and played the ball around and waited for the moments. That's why they were so good.
“We have done it better this season, so we can do it – but we have to do it every game.
“That's why we're all so disappointed, because we expect more and to bring more. It didn't happen but we still have four games to go.”