Hernandez was injured while blocking up field for Julian Edelman. Once Edelman was tackled, he rolled onto Hernandez's lower-leg/upper-ankle area.
Judging by Hernandez's reaction as well as the way he was walking of the field, the injury looks a best a possible high ankle sprain, quite similar to the one that Rob Gronkowski suffered in the AFC Championship Game against the Baltimore Ravens.
As the Patriots know from their experience with Gronk, a high ankle sprain is extremely debilitating to a player performance and judging by Hernandez's reaction the injury could be worse than Gronkowski's.
Gronkowski required surgery after the Super Bowl and had a recovery time of more than two months, which means Hernandez could miss more than half of the regular season if he is to have a similar procedure.
In the short term, this injury severely affects New England's offense as the two tight end set is their preferred formation and it is Hernandez's versatility that makes the formation so effective along with the talents of Gronkowksi.
Hernandez seems to have an unlucky streak going in home openers as he was injured in last year's home opener against San Diego.
The Patriots severely missed Hernandez the following game, a loss to the Buffalo Bills.
If Hernandez's injury is more than an ankle sprain, the Patriots offense could be significantly less lethal for the rest of the regular season.
Hopefully for Hernandez and the Patriots the injury isn't as bad as it looked.
According to the Boston Herald, Hernandez's ankle is not broken but sprained.
Quite a relief as a broken ankle could have resulted in Hernandez missing the rest of the season.