The former British Olympian told the Independent: "When Andriy rang he was just really really thanking me and saying how grateful he was for what I had done for him.
"He was just very, very happy. He was saying 'I feel good. I feel sharp'. The other coaches had been saying he was looking sharp in training, and that is always a good sign, because it would have been easy for me to say that, and for him to believe it, but when other people notice then it means something."
"Bryan rang up a couple of months ago and asked me what I was up to," said Campbell. "He asked whether I could help Andrei sharpen up his sprinting. Before the first session I had been down to Chelsea's training ground and discussed what he wanted from me. But it was made clear to me that Jose Mourinho didn't really want me working with him. Why? I couldn't make a comment on that. I was just told. But then Andriy gave me a call and said could we keep it quiet and do the sessions at his house?
"Andriy lives a few miles away from Chelsea's training ground in Surrey - he has a big garden and a tennis court, and we were able to go through the drills there. He has worked very hard. In the last few games he feels there has been a definite improvement and he has been scoring, so I think our training has worked out.
"Andriy and I really hit it off. We are more friends now than anything else, although there is still the coaching relationship there. He never said a single bad thing about Mourinho. He never said anything bad about any of the other players. He was just a really nice, quiet guy who wanted to get back to his best. And when you come across someone that nice it makes it even more the case that you want to help them and want them to succeed."