George Groves said he was "deeply concerned" to hear that Eduard Gutknecht was taken to hospital and sent his support to the German a day after beating him at Wembley Arena.
Gutknecht reportedly collapsed in his changing room following the fight in London on Friday, when Groves was awarded a unanimous points decision to retain his WBA International super-middleweight title.
The 34-year-old is said to be under observation in the neurological unit at St Mary's Hospital and Groves said his thoughts are with Gutknecht.
"Out of respect to Eduard Gutknecht and his family's privacy I don't want to say too much, but we are all deeply concerned to hear that he was taken to hospital after the fight last night," the Englishman tweeted.
"I sincerely hope it is nothing too serious and I would like to send out all of my support and best wishes to him and his family at this difficult time."