Osimhen fell awkwardly after a collision with Atalanta defender Cristian Romero and hit his head on the field. The Nigeria international was conscious as he was stretchered off and rushed to the hospital.
Napoli released a brief statement saying that "following his injury and head trauma ... he had undergone tests with negative results.''
The southern-based club said Osimhen would remain under observation in Bergamo until Monday. The incident occurred in stoppage time of Atalanta's 4-2 win over Napoli.
Osimhen joined Napoli from Lille at the end of July for a fee of around €80 million. He was injured on international duty in November and then tested positive for coronavirus over the winter break.
The 22-year-old Osimhen only returned to action on Jan. 24 after an absence of more than two months.