The 29-year-old has signed a three-year contract with West Ham and is set to start against United on August 13 when the Premier League bursts back into life.
Hernandez left United to move to Germany in 2015 and is excited about the prospect of returning to Old Trafford.
Chicharito said: "I think I will be a little bit more excited than my team-mates to be back at Old Trafford.
"To start the season with my new team there is important and I will be happy to be there."
Hernandez added: "I was desperate to come back to the best league in the world. It will help me a lot to improve as a player and person. We want to fight to aim for the Europa competitions.
"Slaven Bilic was a massive part of why I joined. Since I joined Bayer Leverkusen he wanted me, he spoke with me. He did everything he could to bring me here but couldn't for a lot of reasons and now I am here I want to repay him."