The Mexico striker made a stunning impact after arriving in England last season, with 20 goals in all competitions, and United have now responded by tying him down to a long-term deal.
His representative Eduardo Hernandez is quoted by various English newspapers as telling ESPN Deportes: "We have renegotiated Javier's contract and he is now committed for five years with Manchester United.
"The initial contract was good for Manchester and for Javier because it enabled them both to see if they could fit together and if he could achieve what was expected of him. Both sides have met their expectations and United were happy to reward Javier after his very good performances last season.
"I will not talk about numbers, but it is a five-year contract. It is a fixed salary with incentives based on the team's success in each of the different tournaments.
"Javier was not badly paid with his initial contract, but it was his first contract and, with respect, not comparable with the likes of Wayne Rooney and Nani. But today I can say that the contract has just been signed and everyone is satisfied."
Eduardo Hernandez also played down recent speculation linking Spanish giants Real Madrid with a move for his client, saying: "It was never likely that Javier would leave.
"Talks began some time ago and United came up with a very good offer and I have nothing but praise for the way they have recognised Javier's performances."
The new deal for Hernandez follows that signed on Monday by United midfielder Tom Cleverley, while striker Danny Welbeck has also been offered a new deal.