Watford are reported to have agreed on a €13m, plus €4m in variables, whilst Barca will receive an undisclosed percentage of any future transfer after the Spain international agreed on a permanent return.
The 24-year-old scored one goal and registered a single assist in seven games whilst on loan at the Hornets for the second-half of the season as his nomadic career continued.
After breaking into the Blaugrana first-team in 2011 as a promising winger, much was expected from the four-time capped Spain international at the Camp Nou.
However, after Barca's recent shortcomings in the UEFA Champions League, the Spanish side have begun their clearout of fringe players in order to fund their incoming transfer dealings for the 2018/19 season.