His current contract with Leverkusen only has a year left to run.
The Mexican - nicknamed "Chicharito" - has impressed at this summer's Confederations Cup and that form, along with his 11 Bundesliga goals last season, has convinced West Ham to make a move.
Hernandez won two Premier League titles during his spell at the 20-time English champions.
Lethal in the box, deceptively good in the air and blessed with a good turn of pace, the Mexican could be just what West Ham need in their search for a prolific goal scorer.
Wayne Rooney in the blue of Everton during Duncan Ferguson’s Testimonial in 2015
Slaven Bilic, meanwhile, is desperate for some firepower to provide the goals to keep West Ham challenging in the top half of the Premier League after a hard spell last season.
However, he was then sent away to Real Madrid on loan after which United sold him to Bayer Leverkusen in 2015 after having netted 59 goals in 157 appearances for the Red Devils.
Slaven Bilic'sside have been dealt a great deal of frustration this summer as their aims to sign Olivier Giroud, Kelechi Iheanacho and Michy Batshuayi have all yet to materialize.
Now 29 years old, Hernandez made a total of 36 appearances across all competitions for Leverkusen last season, scoring 13 goals. Hence, West Ham can benefit from the tension if they are able to match Arsenal's asking price.
FMCG stocks surge to record high, ITC top gainer
Market breadth was in the favour of gainers, with about 3 stocks advancing to every 1 stock that declined. It was followed by M&M 1.60 per cent, Maruti 1.50 per cent and Tata Steel 1.04 per cent.
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