Lauri Love, who has Asperger's syndrome, is appealing against the order signed last week by the home secretary.
The 31-year-old from Suffolk is accused of hacking into the FBI, the US central bank and Nasa.
His US attorney is concerned over how much time would be spent on remand.
Tor Ekeland, who is based in Brooklyn, told the BBC: "Theoretically if he is convicted in the US it is possible he could spend the rest of his life in prison."
Mr Love from Stradishall has said previously he fears he would kill himself if extradited to the US.
Mr Ekeland said: "I think my number one concern is that he would spend a lot of time in pre-trial detention. It could take one to two years to come to trial.
"I have dealt with cases that have taken years."
He said people imprisoned in the US on suicide watch will be put in solitary confinement which he said "makes no sense at all".
"I've had one client stripped naked on suicide watch with the lights on in his cell 24/7," he said.
"The prison system in the UK is more more humane than in the US - it is not a system (in the US) at all interested in rehabilitating people."
Mr Ekeland said he was "obviously disappointed" by the home secretary 's decision, and added: "I am hopeful the High Court (in the UK, which Mr Love is set to make an appeal) will see things differently."
Mr Love was first arrested at his home in 2013.
He had computer equipment seized by British police, but the UK investigation into him was dropped.
The lawyer for a man facing extradition over US government computer hacking claimed he could be detained in jail for years before his trial.