The incident occurred in the 85th minute of the Portuguese top-flight match against Belenenses as Porto searched for a late winner with the scores tied at 0-0.
Nanu, who has been capped by Guinea-Bissau, was chasing a cross into the box before being accidentally wiped out by onrushing goalkeeper Stanislav Kritsyuk.
The defender fell to the ground with players instantly calling for medical assistance.
Play was stopped for more than 10 minutes whilst Nanu was tended to by medics before being carried off the pitch in an ambulance.
Later on Thursday, Porto confirmed the 26-year-old was in a stable condition.
“Nanu had a concussion of the brain and spinal cord injury with loss of memory,” the club tweeted. “At present, he is stable, conscious and aware of time and his surroundings.”
‘He was doing well’
Players from both teams were visibly distressed after the incident, as Nanu lay momentarily motionless on the ground.
“The checks carried out at S. Francisco Xavier Hospital did not reveal any changes in clinical severity. He will continue to be observed at the hospital,” Porto said in a later tweet.
After the match, club captain Pepe said the players wished their teammate a “speedy recovery.”
“The ambulance staff told me he was doing well and that he was already conscious,” he said.