Victory over third-placed Lazio at home would have sent Napoli into the Champions League qualifying round in August, but they lost 4-2 and will instead play in the Europa League.
It could have been so different though if Gonzalo Higuain, who is linked with a big money move away from Napoli this summer, had not squandered a chance to put Napoli up 3-2 with only 14 minutes remaining, but he blazed his penalty over the crossbar and Napoli folded soon after.
It was an ill-tempered affair, with Napoli’s Faouzi Ghoulam and Lazio’s Marco Parolo both seeing red for two bookable offences.
A goal from Parolo just after the half-hour mark gave Lazio the lead, before Antonio Candevra doubled the advantage in first-half stoppage time.
Benitez looked like he had delivered one of his renowned half-time team talks, when his side battled back from the 2-0 deficit at the break.
Higuain almost single-handedly dragged his side back into it with two goals inside nine minutes after the break, before the pivotal moment after 76 minutes had elapsed.
Heads dropped in the Napoli ranks, and Ogenyi Onazi wrecked their hopes with five minutes to go. Miroslav Klose sealed the three points for Lazio in stoppage time.
Benitez is expected to be named the new Real Madrid manager next week with rumors out of Germany suggesting that Jurgen Klopp will spurn the Premier League and replace Benitez at Napoli.