The 29-year-old was signed by the legendary manager in 2010, winning two Premier League titles at Old Trafford, in 2011 and 2013.
Since Ferguson left his post in 2013, United have had four men at the helm, and have not won the league since.
Hernandez, who joined West Ham United from Bayer Leverkusen this summer, told Sky Sports News: "The change of Sir Alex retiring, we knew it was going to be very difficult. He was unique in [the] history of football.
"We all knew there was going to be a transition. You cannot have a manager for 26 years - the best manager in history, achieving everything, changing the squad every couple of years, changing players, even all the staff sometimes, there was always going to be a transition.
"I wasn't surprised. It was normal. Nobody expected when we played for David Moyes that we wouldn't be in European football. But that is part of football."
Chicharito is likely to make West Ham's starting XI as the East Londoners open their 2017-18 account against Jose Mourinho's charges at Old Trafford on Sunday.