Negredo is on the books of Valencia at the moment and has a contract with the Spanish club until June 2019, according to Transfermarkt.co.uk.
Balague has revealed on Twitter that the former Manchester City striker is in advanced talks with Middlesbrough, who got promoted from the Championship to the Premier League at the end of last season.
The Daily Mail had earlier reported of Middlesbrough’s interest in signing Negredo from Valencia in the summer transfer window.
The Spain international scored five goals and created 13 chances in 25 La Liga appearances for Valencia last season.
In three Champions League matches for the Spanish side, the former City striker failed to score a single goal and created four chances.
During the 2014-15 season when he was on loan at Valencia from City, Negredo scored five goals and created 22 chances in 30 La Liga matches.
The former Sevilla striker would be a good signing for Middlesbrough.