In an interview with The Sun, John Harding said Mr Abramovich identified himself to Shaun Pinner, while Mr Harding spoke to Mr Abramovich’s assistant who said the Russian had played a “key part” in their release.
It is understood Mr Harding was set free on Wednesday alongside Mr Pinner, Aiden Aslin, Dylan Healy and Andrew Hill, landing in Britain in the early hours of Thursday morning.
Mr Abramovich announced he would sell Chelsea on March 2, following Russia’s invasion of Ukraine.
The 55-year-old was then sanctioned by the UK Government on March 10, with Downing Street claiming to have proven his links to Vladimir Putin.
“Shaun was chatting away to him about football for a long time and I was speaking to his assistant,” Mr Harding told The Sun.
“She said Roman had played a key part in getting us back. It’s quite extraordinary to think he was involved — and I wouldn’t have even been able to tell you who he was by looking at him.
“He’s well respected by Ukrainians and massively by us now, too — he’s done a hell of a lot for us and we couldn’t thank him enough.
“He was a sound bloke, a really lovely guy. He’s a legend — we absolutely love him and I’m so grateful for his efforts.”
Mr Pinner’s family said he was “in good spirits” and “is looking forward to steak and a glass of red wine” following his release.
A court in the self-proclaimed Donetsk People’s Republic had sentenced Mr Pinner and Mr Aslin to death in July.
They had both lived in Ukraine for a number of years and were serving with its regular forces when the full-scale war broke out.
In a statement released by the Foreign Office, Mr Pinner’s family said: “We would like to thank everybody involved in Shaun’s release, especially all at the Foreign Office, Liz Truss and her team, Boris Johnson and (Ukrainian) President (Volodymyr) Zelensky.
“Shaun would also like to thank the hospitality of the Saudi Government and Prince Mohammed bin Salman al Saud who also assisted in the negotiations for his freedom.
“It’s a very emotional time as you can expect and we are unable to currently discuss so early in his release any details fully. It’s been a harrowing time for Shaun and our family which has now had such a happy resolution. Shaun is in good spirits and still has his sense of humour intact. He is looking forward to steak and a glass of red wine tonight.
“We are now enjoying some family time and would appreciate the privacy at this moment.”
A photo issued by the Foreign Office appeared to show a beaming Mr Pinner with his loved ones.
Three of the British nationals freed by pro-Russian forces were hailed as defenders of “democracy and freedom” by their former commander in Ukraine.
Mr Harding, Mr Pinner and Mr Aslin are believed to have served in the Georgian Legion, a pro-Ukrainian volunteer unit, under Mamuka Mamulashvili.
“All those guys did their best to defend democracy and freedom,” Mr Mamulashvili told the PA news agency.
“They were in the Georgian Legion at approximately the same time… and they were perfect guys.
“It was very important for us to get those guys out of captivity because I’ve been in captivity myself and I know what it is.”
Mr Mamulashvili singled out Mr Harding, a veteran of the British Army, for his contribution to the legion, where he trained younger volunteers as an instructor.
“He is one of the best representatives of his nation who came here to defend Ukraine and I give him my respect,” he said.
Non-profit organisation the Presidium Network, which has supported the family of Mr Healy, told PA that all five had landed safely in the UK and been reunited with their families.
Dominik Byrne, co-founder of the organisation, said: “We don’t know exactly if they’ve all returned back to their homes yet, but we do know they’re with families at the moment.”
The Foreign Office has not commented on the whereabouts of the men.
Mr Aslin’s release was confirmed by his local MP Robert Jenrick.
His mother, Angela Wood, told the BBC that at times she thought she would “never see him alive again”.
“His release happened overnight, I’m still in shock,” she said.
“The first thing I needed to do was to give him a big hug and make sure it was real. It still doesn’t feel real now.”
Ms Wood also said the Russian separatists who were holding Mr Aslin, over a period she described as “torture” for the family, would contact her asking her for money.
A video emerged late on Wednesday of two men sitting inside an airliner, in which Mr Aslin introduced himself and Mr Pinner, adding: “We just want to let everyone know that we’re now out of the danger zone and we’re on our way home to our families.”
Mr Pinner interjected, saying “by the skin of our teeth”, as Mr Aslin continued: “We just want everyone to know the good news etc, so thanks to everyone that’s been supporting us and what not, so it’s really muchly appreciated.”
Mr Pinner added: “Thanks to everybody.”
Prime Minister Liz Truss, who was visiting New York for a UN summit, tweeted: “Hugely welcome news that five British nationals held by Russian-backed proxies in eastern Ukraine are being safely returned, ending months of uncertainty and suffering for them and their families.”