The Italian Serie A leaders suffered only their second defeat of the campaign, with Lazio narrowing the hefty lead down to 17 points.
Matias Vecino netted the winner in the 67th minute, with Spalletti bemoaning his side’s display where they had 14-5 shots from 65 per cent possession, only managing two shots on target.
“There just wasn’t enough quality,” Spalletti told DAZN.
“We passed it around in the messiest way yet, we rushed moves down the flanks where they were allowing us more movement.
“After doing it three or four times, we had the chance to go into the centre, but we lacked patience and didnt attempt enough passes to free the midfielder.
“The midfield was the key to this game, for both teams.”
The defeat ended Napoli’s run of eight straight league games, marking their first defeat since going down 1-0 to Inter Milan on January 4. It was also Napoli’s first home loss in Serie A since April 2022.
Spalletti added: “From what I saw of the attitude and the disappointment when we got back into the locker room, it was a game that went a bit wrong, but nobody here has taken their hands off the wheel.”