The 22-year-old Nigeria international hit his head on the turf after a collision with Atalanta defender Cristian Romero during the final minutes of the game.
"Victor Osimhen suffered an injury to his head ... following the game, he underwent medical tests but they did not yield a conclusive result," the club said in a statement.
"The player will remain in Bergamo until tomorrow (Monday), under the observation of Raffaele Canonico, who leads the medical team at Napoli."
The defeat left Napoli in seventh place on 40 points, three off the top four. Gennaro Gattuso's side welcome Granada in the second leg of their Europa League last-32 tie on Thursday.
