Former boxer Nick Blackwell is in a stable condition in hospital after needing surgery to reduce swelling on his brain following a sparring session.
Blackwell does not have a licence to fight because of his previous injury.
"I couldn't believe Nick would be so stupid as to put his health at risk," said his former trainer Gary Lockett.
"I can't believe the boxer he sparred with, and coach or coaches involved, would ever allow Nick to spar when he's had a brain injury.
"In due course, I hope they are dealt with by the British Boxing Board of Control in the correct manner."
Lockett said he was "shocked" when Blackwell's brother Dan, also a former boxer, told him the news.
He said: "I couldn't believe it. At this time, all our thoughts are with Nick and his family."
Lockett encouraged Blackwell to get his trainer's licence and he had been due to be in Liam Williams' corner for his fight in Cardiff on Saturday but was taken ill after sparring on Tuesday, 22 November.
"We thought that was all going very well and while we knew Nick missed boxing, we obviously didn't realise the extent to which he missed it," Lockett told BBC Wales Sport.
"I am deeply saddened by what has happened. Nick is not just an ex-boxer of mine, but also someone I consider to be a friend."
British Boxing Board of Control secretary Robert Smith says an investigation will begin on Monday.
"What we understand is that Nick Blackwell sparred, he was taken ill and taken to hospital where he is in a stable condition," he told the Telegraph.
"He no longer has a boxer's licence, but he does have a trainer's licence, which is a completely different thing. He should not have been boxing.
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"I'm sickened by it. It's a very tough sport and the doctors here did a wonderful job to save his life.
"He made remarkable recovery and for somebody to go and do what he's done, for what I thought was a very sensible young man, is just beyond belief."
The Daily Mail reported Smith as saying: "We need to find out how this was allowed to happen. I really can't believe Nick has got in the ring again after what had happened, I honestly can't. I don't know what he was doing - it makes no sense."
Eubank Jr, tweeted: "Nick Blackwell is back in hospital, after he tried to start sparring again. He's a tough, tough kid, I know he'll get himself through it again.
"I honestly hope everything is OK and the procedure he's had is not a serious one."