Sir Henry Cooper believes Frank Bruno's apparent breakdown could have been prompted by his problems with coping with life after boxing.
The former British and European heavyweight champion admitted some fighters find it difficult to adjust when their careers finish.
Bruno is undergoing psychological tests at Goodmayes hospital, Essex, after being sectioned on Monday night.
He has been suffering from depression.
Cooper said he was pleased Bruno was getting help.
"If you plan your retirement, you get over it. I don't think Frank really thought it through perhaps," he told BBC Radio 4's Today programme.
Cooper, who captured the imagination of the British public when he fought Muhammad Ali for the world title in London in 1966, said Bruno would have struggled to cope with no strict training regime.
"If you don't have that daily routine any more, where you have to go to the gym at a certain time, train at a certain time, you miss it - all fighters do," he said.
"One day you are a boxer and the next you wake up and you have retired - you are not a boxer, then you think 'what am I going to do now?'
"You have to plan.
"It affects different people different ways, so there's not something that he has or hasn't done.
"If you are going to get that type of mental illness, some people are more susceptible to it than others."
Boxing promoter Frank Warren said he had been worried about Bruno's state of mind for some months.
The boxer divorced from his wife in 2001, and the following year his old trainer George Francis committed suicide.
"He started to surround himself with people he could have done without, they were yes men who were not telling him he had a problem and he had to face up to it.
"It is something he needed to address so now hopefully he can get the help he needs."
Warren also said Bruno had found it difficult to cope after retiring from boxing in 1996, which was what prompted him to throw down a surprise challenge earlier this year to Audley Harrison.
Former boxer Colin McMillan, who has become a boxing manager since retiring, believes the boxing community will rally round to help Bruno.
"I am sure he's going to come through it in a positive light," said McMillan.
"But we need to look at ways in which particular fighters can be counselled when they finish boxing.
"When they retire there's a big void, and you need another direction and focal point."
Bruno confirmed his place as a British boxing great when he lifted the WBC crown in 1995, outpointing Oliver McCall at Wembley Stadium.
"I am sorry to see what happened yesterday because Frank has been one of the characters of the sport and one of our most successful boxers," said the British Boxing Board of Control's general secretary Simon Block.
"He has also been one of our most-loved boxers since the early 1980s and really all we can do is wish him a speedy recovery."