Downes, 24, joined the Hammers from Swansea City last year and has featured sporadically for the London side.
The midfielder has been strongly linked with an exit from West Ham this summer, with David Moyes preferring other options in the centre of the park.
Southampton are set to snap up Downes, with the midfielder heading to the south coast amid James Ward-Prowse having gone the opposite way.
Saints are ready to seal the move for Downes, but have been forced to wait to give him a medical.
The West Ham star is currently ill and will only undergo a medical with Southampton when he has recovered.
Downes is likely to play regularly for Russell Martin’s side this season and his loan deal could contain an option to make the move permanent.
Saints are hoping that Downes’ inclusion in their squad will help their chances of promotion.
And it remains to be seen how the midfielder will contribute to Southampton’s campaign going forward after his arrival at St. Mary’s Stadium.
