A victor of three UEFA Champions Leagues and one La Liga title with Madrid, the 25-year-old is now a pivotal component of Zinedine Zidane's side after initially struggling to establish himself.
They can go move seven points clear of Real and Atletico. "We think we can win this (Europa League) title", Prieto said.
Instead of leaving the club, Isco recently signed a new long-term deal with the club until 2022.
It has emerged that Isco was a target of Barcelona's this summer.
Spanish and European champions Real Madrid began the campaign by winning the continental and Spanish Super Cups to firmly establish themselves as the country's dominant force, but their last two league results have brought them down to earth.More news: Man with knife attacks soldier in Paris
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Bale has scored on each of his three visits to Real Sociedad since joining Madrid, all wins for his side, and he said he hoped this run continued on Sunday.
Real Sociedad has succeeded with its possession-based style of play, taking advantage of skillful players in midfield and forwards who have been capitalizing on scoring chances. "Then we can start being ambitious, and know that we can achieve anything that we want".
The Wales worldwide will be the only member of his team's "BBC" strike force available for Sunday evening's La Liga game at early leaders Real Sociedad, with Madrid missing the suspended Cristiano Ronaldo and injured Karim Benzema.
The 28-year-old, booed inside the Bernabeu in each of Los Blancos' last two home games, was carrying out promotional duties for adidas when targeted by disgruntled fans. We've not got Cristiano nor Benzema, but we will have other players and we'll do well. Against Levante (1-1) we weren't good and we weren't against APOEL in the first half either.