The Hammers are claimed to have made a $500,000 (£382,000) offer to loan Long until the end of the season with a $4.5m (£3.4m) option to buy him outright in the summer.
Speaking last summer, Long said: “As our biggest ally, it seems strange that a football player from the US can’t automatically get a work visa to play in England. Every kid in the USA dreams of playing in the English Premier League. Not many players get the chance, so of course I would like to play there someday.”