Napoli completes their first summer signing with Italian right-back Giovanni Di Lorenzo arriving from Empoli.
Inter Milan must now win their final game at home to Empoli, to guarantee a top-four finish.
Mertens is now the third, tied with Argentina great Diego Maradona, on Napoli's all time scorers list.
De Laurentiis has said he would be happy if Ancelotti stayed at the club "for life" to emulate Alex Ferguson's 27-year reign as Manchester United manager.
Napoli had 67% of possession but could not turn that into chances.
The Italian side have now failed to beat English opponents away from their San Paolo stadium in nine attempts in European competition.
2nd-placed Napoli drew 1-1 at home to 10-man Genoa yesterday.
Despite the trashing Ranieri said he has no regrets about taking the job.
The midfielder will miss the team's Euro 2020 match against Norway on Saturday.
Arsenal face a major battle to reach the last eight after a 3-1 defeat away to Rennes.
The win takes almost certainly wraps up an 8th straight title for the unbeaten Turin giants who have 72 points out of a possible 78 with 12 games left.
Juventus is too far ahead on the scoreboard that Napoli is unlikely to threaten their position anytime soon.
Ancelotti says players played well but ultimately, the lack of scoring is unacceptable.
The Slovakian midfielder made his 520th appearance in Saturday's 3-0 win over Sampdoria.
The big guns suffered upsets with Juventus crashing 3-0 to Atalanta, Napoli falling 2-0 to AC Milan and Roma suffering a 7-1 humiliation at Fiorentina.
The match took place without incident apart from some anti-Neapolitan songs beforehand.
Carlo Ancelotti's side play an Italian league game against AC Milan this weekend before returning to the San Siro next Tuesday for a Coppa quarter-final showdown against Gennaro Gattuso's side.
Lazio are sixth, two points outside the top four places which earn a place in next season's Champions League.
Napoli, with 47 points, open up a 7-point gap between themselves and 3rd-placed Inter Milan.
Match officials in Italy are expected to report racist incidents to public security officials who have the power to stop games.
The incident occurred at the end of a fight between Napoli fans and Inter supporters armed with chains and hammers.
Kalidou Koulibaly was subjected to monkey noises from home supporters throughout the match.
The pressure is on Milan coach Gennaro Gattuso after Federico Chiesa's second-half scorcher pushed AC Milan out of the Champions League places.
Last year's runners-up Liverpool knew victory by two goals or a 1-0 triumph would put them through to the last 16 from Group C