Boxer Eduard Gutknecht was taken to hospital after collapsing in his changing room following his defeat to George Groves on Friday (18 November).
Reports in Germany claim Gutknecht has since undergone emergency brain surgery in a London hospital, an operation have described as "successful."
The 34-year-old lost the WBA International super-middleweight title bout via an unanimous decision at Wembley Arena with Groves comfortably outboxing the German to take a step closer to earning a fourth world title shot.
The German fighter resiliently stood his ground despite the Briton taking control of the fight from the ninth round and suffered heavy swelling around the right eye as the contest wore on.
But after the fight, Gutknecht fell ill in his changing room and was carried out of the arena on a stretcher and taken to hospital. The Daily Mail claim the middleweight "needed resuscitating in the ambulance" before being taking the neurological unit at St Mary's hospital.
German publication Bild now report Gutknecht underwent successful emergency surgery overnight to reduce brain swelling.
In the wake of the distressing news, Groves wrote via his Twitter account: "Out of respect to Eduard Gutknecht and his family's privacy I don't want to say too much but we were all deeply concerned to hear that he was taken to hospital after the fight last night. I sincerely hope it is nothing too serious and I would like to send out all my support and best wishes to him and his family at this difficult time."
Groves' trainer Shane McGuigan added: "Thoughts are with Eduard Gutknecht after tonight's fight."
Groves was awarded the win after the judges' scored 119-110, 119-109, 119-109 in his favour.
Before learning of Gutknecht's condition, Groves told Channel 5: "We want real, big, proper fights. There was no world title on the line, but we knew Gutknecht was going to give me a tough fight. It will push me on for a world title fight and, ideally, I would love a rematch with Badou Jack."