The Mexican striker appeared to catch his studs in the turf,
before falling to the ground in serious pain.
United medical staff attended Hernandez during a four minute
interruption to the game at Villa Park, before placing his left leg
in a splint.
Speaking after the game, boss Alex Ferguson said: "I think he's
torn ligaments in his ankle so he could be out for four weeks."
Villa were also hit with an injury concerns of their own, with
Shay Given and Jermaine Jenas both crocked.
Boss Alex McLeish said: "I'm not sure but I think Shay had a
little tweak in his hamstring. Hopefully it's a grade one (tear)
and will heal really quickly.
"As for Jermaine, we don't know, but we think he's had an injury
to his Achilles but we will wait until the morning to see."
Aside from the injury to Hernandez, Ferguson was satisfied with
the result and especially the performance of goalscorer Phil
Jones.
He said: "We deserved a victory. We had great possession but at
times we were a bit wasteful with chances in the second half.
"Villa Park is a difficult place to go. Phil Jones is a natural
centre-back but he's so versatile he can play in midfield. We've
got options where Phil can play."
Jones was pleased to net his first goal for the club after he
scored an own goal against Benfica in the UEFA Champions League at
Old Trafford last month.
But the 19-year-old is in no rush to determine his role in the
side after being praised for his versatility by his manager.
He said: "I'm delighted. I've been getting some stick for not
scoring and scoring in the back of my own net in the Champions
League was not the best way to open my scoring campaign.
"I'm still young and learning the game and I don't think it
matters if I cement a regular spot in the side at the moment as
long as I keep playing.
"There's time in the future to cement my place in the side and
I'm just happy to play for Manchester United."
United goalkeeper Anders Lindegaard was pleased with the result
after taking over from David de Gea.
He said: "It's very important to keep a clean sheet.
"Clean sheets are the goals for a goalkeeper and in 10 games no
one will remember how many fantastic saves there's been — they'll
remember the clean sheets.
"For my sake I'd rather win 1-0 than 10-1 because I'm a
goalkeeper and want my clean sheet.
"I thought we played brilliantly defensively."