Former special constable Debra Martin previously said Kevin Williams spoke to her in the ground's temporary mortuary.
But, intensive care expert Dr Jasmeet Soar said he had not found "a medical explanation for that".
He said Kevin's heart was likely to have stopped after 15:30 following an unsuccessful resuscitation attempt.
The teenager was one of 96 people who died following a crush at the FA Cup semi-final between Liverpool and Nottingham Forest on 15 April 1989.
Kevin's mother Anne campaigned for more than 20 years for a new inquest into his death, but died before the current hearings began.
Dr Stefan Popper, the coroner who oversaw the original Hillsborough inquests in the early 1990s, ruled all those who died had suffered fatal injuries by 15:15. As such, he said, he did not need to hear evidence on any events after that time.
It was, according to the current coroner Sir John Goldring, a "highly controversial decision", which the bereaved families "strongly disputed".
The new jurors have heard how the 15:15 cut-off meant the first inquests did not look into whether the police, ambulance service and others handled the aftermath of the disaster properly.
They have now heard evidence that some of the 96 may have been alive after 15:15, with one expert witness saying there was a "strong possibility" Jimmy Hennessy was alive on the pitch between 15:15 and 15:20 that day.
Dr Soar, an expert in intensive care medicine and resuscitation, was asked to give his opinion of what happened to Kevin based on the evidence heard about the teenager's movements.
The jury saw how Kevin was carried out of pen three at 15:28, at which point Dr Soar said he was "deeply unconscious", "unresponsive to stimulation" and did not appear to have "any body tone or muscle tone", which was "significant" because it indicated he was not conscious.
Kevin was carried on a makeshift stretcher to the opposite end of the pitch, where a group of Liverpool fans and medics tried to resuscitate him after 15:32.
The jury has heard evidence from Derek Bruder, an off-duty Merseyside police officer who was at the match, who said he found a "slight pulse" in Kevin.
Dr Soar said Mr Bruder may have felt a "weak heartbeat" or he could have been mistaken.
He said a series of photographs of Mr Bruder breathing into Kevin's mouth showed he was carrying out "textbook, exemplary" help, but he was confident that following 10 to 15 minutes of CPR, "there are no signs of life reported by the three individuals, so, yes, he was in cardiac arrest".
In October, Ms Martin told the inquests she later felt a pulse in Kevin inside the stadium's gymnasium, when he was said to have woken up and said "mum".
Dr Soar said he and his colleague Prof Nolan had "discussed at length" Ms Martin's evidence.
"We don't think it's possible to be deeply unconscious for that duration and then to restore an adequate level of breathing and oxygen level and blood pressure for the brain to then regain adequate consciousness, then say a single word and then for that process to reverse again.
"So, we don't have a medical explanation for that."
The inquests continue.
BBC News: Profiles of all those who died

The 3kg (6.6lb) dog is set to become part of a search-and-rescue team used for disasters such as earthquakes.
Its small size means it will be able to squeeze into places too narrow for dogs such as German Shepherds.
Chihuahuas, named after a Mexican state, are one of the the smallest breeds of dog.
"It's quite rare for us to have a chihuahua work as a police dog," said a police spokeswoman in Nara, western Japan.
"We would like it to work hard by taking advantage of its small size."
Momo, aged seven, will begin work in January.
It was not "medically plausible" a 15-year-old Hillsborough victim regained consciousness and said "mum" before he died, an inquest jury heard.
[NEXT_CONCEPT]
A chihuahua named Momo (Peach) has passed the exam to become a dog in the police force in western Japan, in what seems to be a first.