Maradona had been visibly emotional during the match in Saint Petersburg, which Argentina had to win to stay in the tournament.
He leaned over the front of the VIP box from where he was watching with friends and made an obscene gesture after Marcos Rojo scored a late winner.
“I want to tell everyone that I am fine, that I am not and was not hospitalised,” the 57-year-old wrote on Instagram on Wednesday.
A video emerged showing the 1986 World Cup winner looking dazed.
He walked with difficulty and was guided into the dining room of the VIP section by two friends before sitting down in a chair.
“At half-time against Nigeria, my neck hurt a lot and I suffered a drop in my blood pressure,” he said.
“I was checked by a doctor and he recommended that I go home before the second half, but I wanted to stay because we were risking it all,” he wrote.
“How could I leave?”
A medical source told Russia’s RIA Novosti state news agency that paramedics checked Maradona’s blood pressure but that the star refused all other assistance.