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 team will be given access to all the witnesses to the attack, but  the Press Trust of India news agency said they would not be able to interview security personnel.
India accused Pakistan-based group Jaish-e-Mohammad of carrying out the assault in January.
Seven Indian soldiers and six militants were killed in the attack.
This is the first time that Pakistani intelligence and police officials have travelled to India to investigate a militant attack.
The Pakistan team's visit came after Indian Foreign Minister Sushma Swaraj met Pakistan's foreign affairs adviser Sartaj Aziz on the sidelines of a regional meeting in Nepal.
In February, police in Pakistan registered a complaint against "unknown persons" over the Pathankot attack.
An official of the interior ministry of Pakistan had filed the complaint, which mentioned some Pakistani mobile numbers provided by Indian authorities.
The United Jihad Council (UJC) - a coalition of more than a dozen militant groups fighting against Indian rule in Kashmir - said they carried out the attack.
The claim was met with scepticism - the UJC's core members are not known to have carried out attacks outside Indian-administered Kashmir.
Indian security officials instead blamed Jaish-e-Mohammed.
Started by Masood Azhar, the Islamist militant group has been blamed for attacks on Indian soil in the past, including one in 2001 on the Indian parliament which took the nuclear-armed rivals to the brink of war.
The lawyer for a man facing extradition over US government computer hacking claimed he could be detained in jail for years before his trial.
[NEXT_CONCEPT]
A five-member team from Pakistan is in India to investigate a deadly militant attack on an air force base.