Osimhen returned to his Italian club after helping Nigeria reach the Africa Cup of Nations final which they lost 2-1 to hosts Ivory Coast on Sunday.
"According to him and the doctors, the boy is at his lowest," coach Walter Mazzarri told DAZN.
"Coming back from the African Cup of Nations it wouldn't have been the best choice to risk it and let him play for a quarter of an hour to 20 minutes, so the doctor told me."
The tournament took its toll on Osimhen, who suffered stomach pains ahead of the semi-final against South Africa and was only cleared to play the day before the match.
Osimhen scored just one goal in the tournament but his performances played an important role as Nigeria reached the final for the eighth time.
"Today he had an anti-fatigue training, tomorrow he will train. We hope to have him ready for Barcelona (but) his presence will not be certain," Mazzarri said.
"Since I arrived I've never had him. He was injured, I had to recover him gradually. Then there was the Africa Cup of Nations."
