The 22-year-old winger has excelled at West Bromwich Albion this season in the Championship.
He has helped the Baggies close in on automatic promotion this term.
However, West Ham want to integrate the player into their own squad for next term.
It all depends on where they finish, as a relegated West Ham may not be appealing to Diangana for next season.
The player also wants to be in the starting line up regularly, so he may not be willing to accept a squad place where he is part of a rotation.