Redknapp coached Hartson at West Ham United and cannot believe what has happened to the former Welsh international striker.
"I ring every day - I can't believe what has happened to big John," Redknapp told The Daily Telegraph. "He would be right at the top of my list of all-time favourite players that I have ever worked with. He's got a heart of gold, fantastic boy.
"I was on the phone for about 45 minutes to him recently, he is doing his coaching badges, wants to get involved in coaching. I told him to come and spend a couple of days down here with us when he is passing or when he is in London."
And the Spurs boss had a word of warning for all young men: "John had a problem, he had lumps, he had testicular cancer. He thought 'I feel okay' and it has attacked him in other parts of his body. The message to any young man is that you are not too young to get it.
"Any young person should keep checking themselves and, if they do have a
problem, get it sorted. We are all pulling for the big man and if anyone can
pull through it John can - he is as strong as an ox. He is a real fighter."
34 year-old Hartson was capped 51 times for his country, scoring 14 goals, and enjoyed spells at Luton Town, West Ham, Arsenal and Glasgow Celtic.
John Silk, Goal.com