Inter Milan midfielder Joao Mario has arrived in London and is due to start a medical with West Ham.
Sky sources understand the 25-year-old could sign in time to make his debut against Crystal Palace in the Premier League on Tuesday.
West Ham also have the option to make the deal for the Portugal international permanent in the summer for £26m.
Manager David Moyes is keen to sign at least two players before the close of the transfer window having previously insisted he inherited an “imbalanced squad”.
He has since lost attacking trio Andy Carroll, Manuel Lanzini and Marko Arnautovic to injury.
Mario has featured intermittently for Inter following the arrival of Luciano Spalletti as manager and has failed to score in 15 matches this season.
He joined the Italian side for £35m from Sporting Lisbon in 2016 after helping his country to victory at Euro 2016.