The Mexico international was a big hit in his only previous spell in English football with the Red Devils, scoring 59 goals in his four seasons at the club.
As fate would have it, Hernandez begins his West Ham journey with a trip to face United at Old Trafford on the opening weekend of the season, but he is still yet to make a decision on any celebration plans should he find the back of the net.
"I don't know if I will celebrate if I score at Old Trafford," Sky Sports News quotes him as saying. "It's my old club, my old fans, my old stadium but it'll be my first goal for West Ham.
"I am not thinking about the celebration, I just want to have the chance to start and get my team the result. I am very happy and glad, it's the best way to come back and open at Old Trafford.
"I am a Hammer, a West Ham player, I want to do even better than I did in Manchester. I never have regretted leaving United. I had some great experiences there and it helped me when I did move on."
Hernandez joined West Ham from German side Bayer Leverkusen for a fee of £16m on July 24.