Anthony Crolla is "lucky to be alive" after suffering a fractured skull when he was hit by what his trainer Joe Gallagher said was a concrete slab as the boxer confronted burglars.
Crolla, 28, who also broke his ankle in two places, has had his January WBA world lightweight title bout cancelled.
"He was in bits in the hospital," Gallagher told BBC Radio 5 live. "His world title dreams are in tatters.
"The more I think about it, he is lucky to be alive. Unbelievable."
Promoter Eddie Hearn told BBC Sport the Manchester-based boxer was in a "stable condition" and that he was in talks to try and reschedule the world title fight.
Police confirmed that a 28-year-old man was left with a fractured skull after being struck "with a heavy object" when chasing burglars he had spotted leaving his neighbours' home.
Detective sergeant Richard Arthern of Greater Manchester Police said: "Thankfully he is recovering well in hospital."
Trainer Gallagher added: "The next-door neighbours were being burgled, and the burglars came into his garden. He gave chase and apprehended one of them.
"The other one lifted a slab of concrete and hit him with it, and he fell and broke his ankle.
"He is in bits and heartbroken."
Crolla, who has won 29 of his 35 professional fights, will need to have surgery on his ankle, Gallagher said.
Crolla was set for the biggest bout of his eight-year professional career in Manchester on 23 January against WBA champion Richar Abril, 32, from Cuba.
Hearn added: "Unfortunately the 23 January fight is off but our concerns right now are with Ant and his family.
"I'm devastated for him. He has been training for six weeks for this and has a shot at the world title in his home town.
"I have spoken to Richar Abril's team and we are trying to reschedule the fight."