Rahman signed for the club in 2015, playing 23 times in 2015-2016 under Jose Mourinho.
But he did not impress, with injuries and bad form resulting in loans at Schalke 04, Reims, Mallorca, PAOK and Reading.
"I am sad things didn't work out at Chelsea," the PAOK defender told BBC Sport.
"I just had so many bad injuries whilst I was there, and because I wasn't 100% fit I wasn't able to give my absolute best."
"I owe them a lot because it was Dr Paco (Biosca, ex-Chelsea medical director) who convinced me not to retire after my second ACL injury," Rahman added on the serious injury he sustained in 2019.
"It had taken me 18 months to come back and when the doctor told me I had another ACL injury I decided that I had had enough," he recalls. I told my family I was quitting.
"But Dr Paco said that he had seen other players come back and promised that if I was patient and mentally strong that I could do it too. I slept on it and then decided I wanted to fight for my career. I am grateful that football has given me a second chance. I'm just trying to enjoy it to last moment."