Since joining United in 2010, the 24-year-old striker has seen his summer breaks disrupted by international commitments - first at the World Cup and then the Gold Cup.
After seeing Hernandez net twice in a 3-2 UEFA Champions League victory over Braga on Tuesday, Ferguson said: "The thing that is making the difference is he got a proper rest in the summer.
"It wasn't exhausting in the way it had been in the previous three years.
"He was tremendous the other night.
"He is such a dedicated footballer and professional in every training session. He's a really terrific player and the good thing is that he is still improving."