John Clark, 58, a forensic pathologist at Glasgow University, carried out a post-mortem of Barry Fleming, 28, of Broxburn on 30 May last year.
Mr Fleming had been found dead outside the Clifton Arms pub, in the town, the day before.
Mr Clark told the High Court in Livingston yesterday that Mr Fleming had died from a stab wound, and had suffered "bleeding internally".
Mr Clark was giving evidence on the third day of the trial of Christopher Bouhadi, 25, who denies murdering his then flatmate, Mr Fleming, in Broxburn's Station Road on 29 May last year.
He said: "The man has died from a stab wound. The knife went through the right side of his neck, and through his chest to the back left of his chest. He was bleeding internally. He would have died quickly and couldn't have been saved."
The trial continues.