The former Manchester United and Real Madrid striker completed his £16m move to the London Stadium yesterday, joining on a three-year deal from Bayer Leverkusen, where he scored 39 goals in 76 games.
Hernandez is West Ham's fourth signing of the summer after Joe Hart, Pablo Zabaleta and Marko Arnautovic, and Sullivan is "optimistic" that the quartet will help them to enjoy a successful 2017-18 campaign.
"I think he is potentially the best player who has ever come to the club. In recent years, he is probably the best proven goalscorer we have signed since we have been at the club," he told talkSPORT.
"He's done it in the Premier League with Manchester United. We were lucky he only had a year left on his contract with his club and that allows you to negotiate a deal. We tried very hard to get him two years ago but unfortunately he chose the German club ahead of us.
"We are very optimistic with the four players we have signed, we think they will make a huge difference. I can't comment on players from other clubs but what I would say is midfield is not a position we are looking for. We have a lot of good midfielders at the club, but never say 'never'."
Hernandez's full debut for West Ham could come against former club Manchester United at Old Trafford on the opening weekend of the new Premier League season.