Mario Suarez has completed his Watford medical and is now expected to sign for the club in a €5m (£4.1m) deal from Fiorentina.
Sky sources understand Suarez, who arrived in the UK on Wednesday night, will join on a three-and-a-half year contract.
Hornets boss Quique Sanchez Flores worked alongside the Spain international when the pair were at Atletico Madrid, the club he joined Fiorentina from last summer.
Suarez has struggled to make an impact in Serie A, starting just five matches, but Flores said: "Mario Suarez is close. It is not finished. At the moment I don't have official notice off the club that it is done."
Suarez has won three caps for Spain and during his second stint for Atletico was a key player in helping Diego Simeone's side win La Liga and reach the Champions League final in 2014.