Hernandez is set to miss Sunday’s sell-out fixture with an injury sustained towards the end of Leeds’ 3-2 victory over Norwich City before the international break.
The influential Spaniard pulled up towards the end of a pulsating contest at Carrow Road - a match which saw United climb into the Championship’s play-off zone - and was substituted on 89 minutes.
Despite Leeds’ attempts to nurse him back to full fitness during the past two weeks, head coach Garry Monk said Hernandez was “unlikely” to feature against Newcastle and would undergo fresh scans in the next 72 hours.
Hernandez, the former Valencia and Swansea City playmaker, has been central to Monk’s plans this season and central to the run of form which turned Sunday’s game at Elland Road into a top-six match.
His absence could see 18-year-old Ronaldo Vieira come into the starting line-up on the back of his dramatic injury-time winner at Norwich.
“Pablo suffered a hamstring strain in the game against Norwich so he’s a doubt,” Monk said. “We have to assess him but it’s unlikely he’ll be involved. Everyone else is ready to go.
“We’re still not sure (how long Hernandez will be out) but we’re going to have another scan to get the full report on him.”
Midfielder O’Kane, however, has handed Monk a major boost by returning to training from the groin strain which forced him to cut short his international duty with the Republic of Ireland last week.
Winger Stuart Dallas, who pulled a calf muscle while away with Northern Ireland at the start of October, has also put himself in contention by completing two weeks of work at Thorp Arch.
O’Kane’s recovery is particularly important for Monk with captain and midfielder Liam Bridcutt still absent with a fractured foot.
Monk has contended with his share of injuries this term and Bridcutt’s spell on the sidelines will extend into a 12th game this weekend. Dallas’ comeback, however, ends a six-match absence.
Monk said: “We’ve not got the biggest squad or the most experienced squad but I trust all my players. I’ve proven that. I’ve used all of them and they’ve all contributed to the season so far.
“Stuart was ready to train at the start of the international break so he’s had a good period now. He’ll be in contention. Eunan’s fit and he’s been training so he’s ready to go for the weekend.”
Newcastle have avoided a headache of their own after midfielder Jonjo Shelvey avoided an immediate ban by pleading not guilty to a Football Association misconduct charge this week.
Shelvey will face a minimum five-game suspension if found guilty of abusing Romain Saiss during a match between Newcastle and Wolverhampton Wanderers in September. Shelvey denies any wrongdoing and a disciplinary hearing will take place at a later date.
Monk played with and coached the England international at Swansea City.
“Jonjo’s been excellent this season,” said Monk. “He’s got fantastic ability.
“When we had him at Swansea and I was working with him, the big stage was where he wanted to be. Helping him get back into the England squad at the time - he thrives of that type of feeling.
“He’s a dangerous player for them but they’ve got a number of dangerous players and an expansive squad - high calibre in this league. We’re going to be tested but my young players will be ready.”