The Spanish goalkeeper returned to Sevilla following PSG's league triumph on Saturday evening. After winning the title, the club gave their players permission to travel wherever they wished.
The incident occurred while Rico was horse-riding but his entourage have indicated it was not a fall from a horse. It is believed Rico was hit in the neck by a runaway horse.
He was airlifted to the Virgen del Rocio hospital where he is set to undergo medical exams and is surrounded by his family and friends. A PSG spokesperson told the BBC: "He is in a serious condition."
His former club, Sevilla, have confirmed the news and sent their best wishes. "Lots of strength and speedy recovery," the Europa League finalists wrote on Twitter.
PSG also issued a statement of their own, writing: "Paris Saint-Germain learned of the accident involving its player Sergio Rico on Sunday and remains in constant contact with his loved ones. The entire Red and Blue community offers them its full support."