Osimhen, who had been angered by the club following a social media post this week, doubled Napoli’s lead early in the second half. Leo Østigård, Gianluca Gaetano and Matteo Politano also got on the scoresheet.
Napoli moved third, four points behind league leader AC Milan, which beat Lazio, 2-0, following a goal from United States standout Christian Pulisic. Inter Milan can move back level with its city rival with a win at Salernitana later.
Pulisic netted his third goal for Milan but his first in more than a month.
Milan’s pressure had been building, and the hosts finally made the breakthrough on the hour mark. Tijjani Reijnders slipped the ball through to Rafael Leão and he crossed for Pulisic to fire in on the half-volley with the inside of his left boot.
Pulisic was given another standing ovation by the San Siro crowd when he was substituted late on.
Fellow American Yunus Musah almost doubled Milan’s lead in the 73rd minute, but he was denied by a superb save by Ivan Provedel.
The Lazio goalkeeper had pulled off an equally impressive stop at the end of the first half to prevent Olivier Giroud from marking his 37th birthday with a goal. Reijnders backheeled the rebound onto the post.
Noah Okafor sealed the match two minutes from time with a simple tap-in after some great work by Leão.
Lazio thought it had pulled one back in stoppage time with a curled effort from Pedro, but it was ruled out for offside.
Napoli had ended a three-match winless run with a 4-1 win over Udinese midweek, but coach Rudi Garcia opted against starting Osimhen on Saturday ahead of Wednesday’s Champions League match against Real Madrid.
Napoli took the lead in the 16th minute when Piotr Zieliński floated a free kick from the left to the back post and Østigård headed it into the top corner for his first Serie A goal.
Garcia brought on Osimhen at halftime and the forward had an instant impact as he found the back of the net just six minutes later. Much of the credit has to go to Khvicha Kvaratskhelia, who cut in from the left and lifted a perfect ball onto the head of Osimhen at the back post.
It was Osimhen’s fifth goal in seven league matches this season. The 24-year-old was the top goalscorer in Serie A last season.
Gaetano was brought on for his season debut with seven minutes remaining, and he scored shortly afterward with a strike from the edge of the area.
He also won a penalty after being tripped by Ylber Ramadani. Politano, who had been given the ball by Osimhen, struck it into the bottom left corner.
Reporting by The Associated Press.
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