Edinson Cavani's hat-trick helped inflict a decisive defeat on Zeman's side, who had returned from a Christmas break in Florida just days before the fixture.
But the Roma boss said it was his team's lack of cutting edge in front of goal that cost them, not their tiredness.
“I do not think jet-lag had any influence. We created six or seven chances, more than our opponents, but we did not convert them and they did, so that is football," he told reporters after the game.
“We knew Napoli played on the counter-attack and they managed it straight away with a fast move. In my view they scored some great goals and we have to applaud them."
Despite the loss, Zeman was happy with the performance.
“It is a step backwards in terms of result, but the team tried to play and put the opposition under pressure,” he said of the defeat.
“Of course goals count in football, we didn’t score them and Napoli did. I am generally happy with the performance, though."
He also had praise for Cavani, saying: "He is very quick, certainly quicker than Burdisso and Castan, and it is criminal to leave him in one-on-one situations.”