He has been in jail since Tuesday, when he was remanded in custody after denying the murder of Mr Magee jnr.
The Prison Service said the jail attack is being investigated by the police.
Mr Koca's defence solicitor Eamon O'Connor said his client was attacked after leaving his cell for a family visit.
"He was going from the cell to the visiting area. Two other prisoners were speaking with him. He turned around and then he was hit in his face," Mr O'Connor said.
The lawyer said that as well as the fracture and stitches, Mr Koca suffered "possible nerve damage" with a loss of feeling around his mouth.
Mr O'Connor said he spoke to his client on Friday morning and he was "not able to communicate very well" because of his injuries.
He said that Mr Koca attended his family visit immediately after the attack "to make sure that someone saw the injuries that he sustained". He was then brought to hospital, before being returned to Maghaberry.
The lawyer said Mr Koca has requested in writing that prison authorities move him to an area where he could be held in "complete segregation" for his own safety.
He said it was his understanding that the assault was captured on CCTV and was reported to police. Mr O'Connor said there was concern over how it could have taken place, as prison authorities have a "duty of care" to all inmates.