Osimhen picked up a second yellow card in the 86th minute as nine-man Napoli suffered a 2-0 defeat to AS Roma on Sunday.
Osimhen was expecting to justify his contract extension for Napoli but it ended a bad day in the office for him.
Osimhen was booked in twice in the second half after Napoli players, Politano was earlier shown a second yellow cars for a bookable offence.
Goals from Lorenzo Pellegrini and Romelu Lukaku, while Matteo Politano and Victor Osimhen were substituted for the visiting team.
After just five minutes on the pitch, Pellegrini scored the first goal in a nerve-wracking match in the 76th minute with a half-volley into the top corner.
Champions After Politano was sent off in the 66th minute for kicking out at Nicola Zalewski after the Roma player pulled him back by his jersey, Napoli was already down to 10 men.
Roma moved up to sixth place on 28 points, one ahead of Napoli who dropped to seventh.