Psychologist Jonathan Scholtz said Oscar Pistorius has "almost given up" and should not serve a custodial sentence.
Defence witness Jonathan Scholtz argued that a "broken" Oscar Pistorius should be admitted to hospital rather than return to prison on day one of the athlete's sentence hearing
Olympic and Paralympic sprinter Pistorius was released on house arrest last October after serving a fifth of his five-year sentence for killing his girlfriend Reeva Steenkamp.
The Supreme Court of Appeal ruled last December that his initial conviction of culpable homicide should be changed to murder - which carries a minimum jail sentence of 15 years.
Pistorius, who claimed he thought Ms Steenkamp was an intruder when he shot her dead, returned to Pretoria's High Court on Monday for the start of a hearing which could take several days.
Psychologist Scholtz stated that the 29-year-old should not serve a custodial sentence due to his mental condition, as he suffers from post-traumatic stress disorder, agoraphobia, depression and generalised anxiety disorder.
"I feel unfortunately his condition has worsened. He really has almost given up, his spirit seems broken," Scholtz, who said he does not feel Pistorius is fit to testify this week, told the court.
"It is my considered opinion prison would have a detrimental effect on Pistorius as a person.
"It would not be constructive. It would be better if he gave back in constructive ways - using his skills to enhance the lives of others [carrying out community service]."
Chief prosecutor Gerrie Nel questioned why Pistorius was unable to testify this week after he did manage to conduct a television interview for ITV.
The defence is to call a second witness in the hearing, which was adjourned until 9.30am local time on Tuesday.