Barry McGuigan suffered a stroke during the early stages of his management career with Carl Frampton, a court was told on Thursday.
Mr McGuigan revealed he experienced the medical condition in April, 2013 - the same year Cyclone Promotions - which was overseen by himself and members of his family - was set up.
The retired ex-professional boxer also said his former protege never discussed any issues he may have been having with him directly before their working partnership came to an end.
Mr Frampton (33) is suing over the alleged non-payment of up to £6m in purse fees, broadcasting rights, ticket sales and merchandising during their partnership.
Both Mr McGuigan and Cyclone Promotions are named as defendants. A counter-suit for breach of contract has been filed against the boxer. Both men deny any wrongdoing.
Mr McGuigan insisted Mr Frampton was "one of the family", telling the court Mr Frampton was made a director of his company so the fighter could see that everything was "open and honest".
"We were very open. The (Cyclone) office was above the gym," he told the judge.
"We wanted him to come and see anything that he wanted to see."
He also insisted a $1.6m deal struck for Mr Frampton's 2016 fight in New York against Leo Santa Cruz - of which Mr Frampton had been told he would receive $1.5m (£1.15m) - "far exceeded anything else that had been on offer".
On Wednesday Mr McGuigan denied accusations he had ripped up a $500,000 (£383,000) cheque presented to Carl Frampton following his 2016 win against Leo Santa Cruz so he could “control or conceal payments” to the fighter.
He dismissed the accusation as "absolute cobblers", insisting the cheque was handed back to the US promoters because it “wasn’t the true figure and he (Frampton) knew that”.
On Thursday the court was also told how Barry McGuigan could have secured Carl Frampton's dream of fighting for a world title at Windsor Park Stadium if they had stayed together.
Mr McGuigan also claimed the boxer was already negotiating an exit before he walked out on their relationship.
Completing six days in the witness box, the boxing manager said of Mr Frampton: "He was like one of the family."
Their split came after an ill-fated scheduled fight against Andres Gutierrez in July 2017.
That contest was ultimately called off when the Mexican slipped and injured himself in the shower on the eve of the bout.
Asked by his barrister, Liam McCollum QC, if he had any plans to severe ties with Mr Frampton at the time, Mr McGuigan replied: "No."
Instead, he said, the intention was to rearrange a fight against the same opponent that November and then set up a third contest with Leo Santaz Cruz the following spring.
Three potential locations were suggested: New York, Las Vegas or Windsor Park Stadium in Belfast - a venue Mr Frampton has often described as a dream venue.
During cross-examination on day 14 of the case, counsel for Mr Frampton challenged him on a proposed purse fee of £500,000.
"If that's a Belfast fight, that's an awful lot more than any purse you ever got for Mr Frampton in Belfast before, isn't it?" Gavin Millar QC contended.
Mr McGuigan replied: "We would have tried to put that on in Windsor, it would have been April-May time, that would have been an open air show, that would have been the objective."
He disagreed that his plans were all based on speculation.
"I'm pretty sure Carl would have beaten this guy (Gutierrez), would have looked good in doing so, and then would have had an opportunity to fight Leo Santa Cruz a third time, either at home in Windsor, or in New York or in Las Vegas," he said.
Part of Mr McGuigan's counter-claim involves an alleged loss of commission for the cancelled Gutierrez fight.
Mr Millar pressed him on how the boxer was liable for a contest that was called off.
"Because he walked out on his contract," Mr McGuigan said.
"I believe that he was negotiating leaving, and he was in the throes of walking away from us.
"He broke a contract where we had worked very hard for him for a long time, and did a magnificent job with him."
Mr Millar argued, however, that the claim was not due to his client's actions.
"It was Mr Gutierrez injury that caused that loss of commission," he submitted.
Mr McGuigan replied: "Mr Gutierrez injury caused the fight to be cancelled, yes."
The boxing manager was also quizzed about Mr Frampton's purse of around £145,000 for defeating world champion Kiko Martinez to take the super-bantamweight title in September 2014.
The Spaniard was paid $700,000 for the contest staged at Belfast's Titanic Quarter.
Insisting it had been "a very good purse" for Mr Frampton to secure a world title fight on home turf, Mr McGuigan said it was common for a challenger to be paid less.
Recalling his own boxing career when he famously defeated Eusebio Pedroza to become world champion in 1985, he said: "I fought for the world title, 27,000 people in Loftus Road, got £90,000 and Pedroza got £600,000.
"That regularly happens, the champion comes to the challenger's back yard, he gets paid lots of money and the challenger, for the opportunity to fight at home and a big chance of winning, gets paid considerably less."
The court heard Mr McGuigan negotiated a $1m purse for Mr Frampton to defend his title against Alejandro Gonzalez in El Paso, America.
"If you look at Carl Frampton's purses, and you look at the super-bantamweight division, in its history there has rarely been anybody that has earned more money than Carl Frampton," he said.
"That is something I'm very proud of. Those purses were extraordinary."
Before finishing his evidence, Mr Justice Huddleston asked if he ever directly approached Mr Frampton about their deteriorating relationship.
"We had many conversations," Mr McGuigan told him.
"The interesting thing was he never once said to me 'I'm unhappy', there were multiple opportunities for him to say that.
"I could have had a discussion, I'm a very good person and a very decent person, and I would have been able to resolve any problems he had, had he come and spoken to me about them, which he didn't."
The case was adjourned to next month.