Sir Alex Ferguson has admitted Hernandez has been lucky to emerge from a wild tackle by Ashley Cole unscathed.
The Scot feared the worst when Hernandez was caught by Cole during the second half of Sunday's 3-1 Premier League win over Chelsea at Old Trafford.
Hernandez limped off and it appeared he was set for an extended period on the sidelines.
However, the Mexican returned to training on Thursday and is in contention for Saturday's trip to Stoke.
"Hernandez is very lucky," Ferguson said.
"At the time he had no feeling in the leg. There was numbness and we thought there might have been some nerve damage.
"But he trained very well yesterday and is available for tomorrow."
Ferguson branded Cole's challenge reckless in the aftermath of the game, although referee Phil Dowd decided it was only worthy of a yellow card.
The only conclusion the Scot could draw from that decision was the officials did not see it, and he is not entirely sure why his side did not get a penalty either.
"I am confused," he said. "I don't think the referee could have seen it.
"It was a spur-of-the-moment challenge by the player, reckless yes.
"But I don't know how the referee could have possibly seen it and the linesman wasn't in any better position.
"I don't know what the rules and regulations say about that when the ball is out of play - there seems to have been some discussion about it not being a penalty because the ball was out of play.
"I wasn't aware of that - I don't think anyone was."
Ferguson is equally bemused by the state of Vidic's fitness.
The Serbian limped out of the opening-day win at West Brom with a calf injury.
At the time, it was estimated Vidic would be out for six weeks.
However, that deadline is now approaching and the defender is no nearer a comeback.
"It's slow progress with Nemanja," revealed Ferguson.
"He's not done any training. He is still getting treatment on the calf.
"Calves can be troublesome. I remember when Bryan Robson had calf trouble it kept him out for three months.
"Hopefully it's not as long as that, but he's not making any great progress at the moment."