Ernesto Valverde was appointed the new manager of Barcelona following the departure of Luis Enrique at the end of last season, and he immediately made the Red Devils midfielder one of his top summer targets.
Herrera worked with Valverde at Athletic Bilbao before he moved to Old Trafford in 2014, and the manager is keen to reunite with his former player at the Camp Nou. The United midfielder however has admitted that he is happy at Old Trafford and has also been touted as a future captain once Wayne Rooney relinquishes the armband.
According to Spanish publication Marca, the Portuguese manager has issued a firm stance on the Spain international's potential transfer 'he is not for sale at any price'. And despite his contract expiring next summer, United will not allow him to leave and have reportedly made him a fresh offer worth £120,000-a-week ($153,000).
The 27-year-old had a difficult start to life in the English top-flight, but found his best form under Mourinho in the recently concluded campaign. Herrera was played in a deep lying midfield alongside Paul Pogba and he put in impressive performances on a consistent basis throughout the campaign.
The Spanish midfielder was named United's player of the season, and was also named the man of the match following their 2-0 win over Ajax in the final of the Europa League.
Valverde is keen to sign a midfielder this summer and had made Herrera his first choice signing. Marco Verratti was another option for Barcelona, but the Italian midfielder has also suggested that he will remain with current employers Paris Saint-Germain next season.
Barcelona, meanwhile, have identified a third alternative in Jean Michael Seri in the event they fail to sign their primary targets. The Ivory Coast international impressed for Ligue 1 side Nice this season, and the club's president confirmed that the La Liga outfit and PSG are interested in signing the box-to-box midfielder.