Isco is important for Real Madrid, La Liga and Spain despite his dip in form, according to Fernando Sanz.
The 26-year-old playmaker is yet to start a match in the league since Santiago Solari replaced Julen Lopetegui at the helm, with reports suggesting the pair have a fractious relationship.
Isco was jeered during Madrid's 3-0 Champions League defeat to CSKA Moscow last week and appeared to respond with an angry gesture towards the crowd, though Solari claimed the player was simply claiming for a foul or corner.
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Former Madrid player Sanz, who won La Liga and the Champions League with the club, acknowledged it is tough for players to consistently perform at the expected levels, but backed Isco - who has been linked with a January move away - to turn things around and prove his worth.