With one eye on their Champions League game against Real Madrid on Tuesday, Napoli manager Rudi Garcia left Osimhen out of the starting lineup, but the Nigerian striker wasted no time once he replaced Giovanni Simeone for the second half.
After a loose ball from Lecce allowed Osimhen to gain possession in his own half, his pass sent Khvicha Kvaratskhelia down the left wing before the Georgian cut in and his perfect cross was headed home by Osimhen in the 51st minute.
The visitors had taken the lead when defender Leo Ostigard scored his first Serie A goal while Gianluca Gaetano added a third two minutes from time when he scored from distance. Matteo Politano completed the rout with an injury time penalty.