The exact reasons for each player are not known, with Vidic a particularly surprising exclusion, but injuries and a need to rest will have been factors in choosing the squad.
As a silver lining, several youngsters make the cut: Fabio is included after a disappointing loan to Queens Park Rangers last season, as are academy stars Michael Keane, Adnan Januzaj and Jesse Lingard. United announced the full squad on their new Twitter account:
The inclusions of the youngsters is highly promising, at least. Hopefully we'll get to see plenty of Wilfried Zaha, and Adnan Januzaj is a hugely exciting young talent. Jesse Lingard is also a nice surprise, while Michael Keane was probably expected, although will likely be loaned out for at least some of the season.
One thing we should mention: don't read too much into the absences and inclusions here. Players attending pre-season training or tours rarely means much. The fact Hernandez is absent is probably to do with his escapades for Mexico this summer, rather than any suggestion he might be leaving the club. Similarly, just because Wayne Rooney is included doesn't mean that he's 100% guaranteed to be staying.