Diamanti has been on the verge of a move to Watford since last week, and head coach Quique Sanchez Flores has confirmed to BT Sport that the Italian is training with the club, but that he may not be available for Saturday’s game with West Brom.
Flores said: “Will the move be completed in time for Saturday? It is difficult for Saturday but it's close. He’s training with us every day, he's performing well - the teams are talking to arrange the last details and he has the possibility to be incorporated onto the list.”
Diamanti will be Watford’s 11th signing of the summer when the deal is completed, and the attacking midfielder brings Premier League experience to a side lacking in relation to many of their rivals.
The 32-year-old spent one season with West Ham in 2009/10, becoming something of a cult hero after scoring seven goals in 28 Premier League games.
The Italian international spent last season with Guangzhou Evergrande - as well as a short spell on loan with Fiorentina - and it is from the Chinese club that he is set to move to Watford.
He has been training with the Hornets for over a week, but the paperwork for his season-long loan move has yet to be finalised.
Flores does not believe that the move will be confirmed in time for Diamanti to make his debut for the Golden Boys on Saturday, with West Brom travelling to Hertfordshire to take on the newly promoted side.
However, the Hornets have been handed a boost in the attacking ranks, with both Almen Abdi and Steven Berghuis expected to be fit to make their first appearances of the 2015/16 campaign against the Baggies.