Dominican Republic fighter Lenin Castillo was left unconscious and shaking on the canvas after suffering a brutal second-round KO against Callum Smith in their bout in London.
Fighting on the undercard of the blockbuster heavyweight showdown between home hero Anthony Joshua and Ukrainian star Oleksandr Usyk at the Tottenham Hotspur Stadium, Smith and Castillo contested a light heavyweight bout which ended in brutal fashion in the second round when Smith caught his rival with a big right hand to the temple.
The blow sent Castillo onto the canvas, where his head rested against the ropes while his legs twitched uncontrollably.
The referee immediately waved off the fight as medical staff entered the ring to attend to the floored Castillo.
The 33-year-old was stretchered from the ring and was reportedly conscious and talking, but was rushed to hospital for further checks.
Online, fans sent their well-wishes to the Dominican star – with one fan calling the knockout “the scariest I’ve ever seen.”
Rock star and former Oasis frontman Liam Gallagher seemed shocked, adding a kiss as he wrote: "Hope Lenin Castillo is OK. Heavy sh*t, man."
Matchroom promoter Eddie Hearn updated fans by letting them know that Castillo was “responsive” but described the knockout as “brutal”.
Smith picked up a win on his light heavyweight debut in North London, bouncing back from his unanimous decision defeat to Mexican great Saul ‘Canelo’ Alvarez in their WBC super-middleweight title fight in Texas last December – which remains Smith's only defeat in 28 pro fights.
"That's never nice to see... especially when it's you involved in it," the Liverpudlian told The Sportsman afterwards.
"It's a sport at the end of the day and I always just hope that me and my opponent leave the ring safely.
"I've heard he's responsive so, fingers crossed, everything's OK – but no, it's never nice to see. Obviously I wanted to win but you never want to win that way."
Castillo’s worrying loss on Saturday night was his fourth in 26 professional outings, but fans will simply be hopeful that he makes a speedy recovery from his horror KO.
Also on rt.com ‘This guy’s got a lot of demons’: UFC boss Dana White reacts to Jon Jones’ latest arrest (VIDEO)