The striker did not fly with the squad from Abidjan, where they beat Angola in the quarter-finals on Friday, to Bouake where the first of Wednesday’s semi-finals will be played.
"Team medics confirmed that he has been placed under close watch with a member of the medical team staying behind in Abidjan with him. If cleared by tomorrow morning, he will join the rest of the squad before 5 p.m," a Nigerian Football Federation statement said.
Osimhen's absence would be a major blow to Nigeria, for whom he has been their outstanding performer at the tournament with his endless running and phenomenal work rate.
The Bouake clash is followed on Wednesday by the second semi-final in Abidjan in which hosts Ivory Coast face the Democratic Republic of Congo.