Sir Mo Farah’s former PE teacher Alan Watkinson said the athlete had “no other option” but to “lock away” his past during his rise to greatness.
A BBC documentary, The Real Mo Farah, revealed the four-time Olympic champion was trafficked into the UK aged nine under another child’s name.
Sir Mo first confided in Mr Watkinson, who then helped him find a foster family and obtain British citizenship.
Speaking on Good Morning Britain, Mr Watkinson was asked if it was difficult to watch the athlete sell a story that was not true as his profile grew.
He said: “It was really difficult, but he was in that situation where, really, he had no other option.
“I think the progress he was making as an athlete, the knowledge that if this story came out at any particular time, what would that mean for him?
“I believe he locked that away because he wanted to focus on the thing that drove him.”
Following the film’s broadcast on Wednesday, Mr Watkinson received an outpouring of love for helping his former pupil.
Speaking about the “secret” he held for many years, Mr Watkinson said: “I know a lot of his friends were saying to him, ‘Are you sure? Do you really want to do this? Can’t you just leave it alone? You’ve got a really nice life; this could cause you all sorts of problems.’
“But I know Mo. When he decides he wants to do something, he goes and does it, and more often than not his instinct is right.
“I knew from what he said to me he had to do it. It was something he was very, very exercised about.”
Mr Watkinson said Sir Mo “came alive” during PE lessons because it was “the one enjoyment he had” and the “one way he could express himself”.
He said: “I saw a very different side to him than other people saw. He was quite aggressive in his first year at school. He had no access to the curriculum because he couldn’t speak the language and he had a really, really difficult time.
“There is no way I would have guessed, maybe naivety as a young-ish PE teacher, and it was a real bombshell.”
He said Sir Mo “has always been” the person portrayed in the documentary.
“I have known for a considerable part of his life he is such a generous and lovely guy. You always forgive him everything because he turns up with this big smile on his face and he is such engaging company, and I think you will probably see more of him now. The guard will be lifted,” Mr Watkinson said.
He does not think Sir Mo is “looking for retribution” following the launch of a police investigation into his trafficking, he added.
