Hernandez, 24, was an instant hit at United, firing 20 goals - including 18 in 20 starts - after signing from Chivas for £6million two years ago.
Chicharito said: "This will be my first complete pre-season because during the last two summers I met up with the team in America during the tour.
"It will be important and I just want to enjoy it and do my best. I know I need to keep learning - you never stop learning - and I want to continue to improve and work hard in order to repay the people that put their trust in me.
"I know I need to be professional on and off the pitch and I want to make sure I'm ready for the new season.
"I really enjoyed my summer break in Mexico with all my family and my friends but I have to say I was missing not being at Carrington. I'm very happy to be back and to see my team-mates and start pre-season."