Lenin Castillo was "responsive" having been rushed to hospital after being knocked out by Britain's Callum Smith on the undercard of Anthony Joshua's world heavyweight title fight against Oleksandr Usyk in London on Saturday.
An overhand right from the Liverpool boxer midway through the second round struck Castillo flush on the temple and saw referee Bob Williams halt the contest as soon as the Dominican had hit the canvas.
Smith (28-1, 20 knockouts) had jumped onto a rope following his victory before cutting short the celebrations when he realised just how severely he had hurt Dominican Republic fighter Castillo, whose legs appeared to be trembling as paramedic treated him promptly inside the ring at Tottenham Hotspur Stadium.
Castillo, 33, was attended to where he lay for several minutes before carried away on a stretcher to sympathetic applause from spectators watching the light-heavyweight bout.
Promoter Eddie Hearn wrote on Twitter: "A horrible brutal knockout. Castillo is responsive and on his way to hospital."
This fight was the 31-year-old Smith's first at light-heavyweight, having suffered the inaugural defeat of his professional career in December last year when Saul 'Canelo' Alvarez convincingly deprived him of his World Boxing Association (WBA) super-middleweight title.