The former Germany midfielder has told warned the Catalan club that they must win Saturday's Clasico if they are to stand any chance of winning the league
The reigning champions approach this weekend's game four points behind their fierce rivals, with just five games until the conclusion of the season.
The Catalan club were victorious when the two sides clashed in the Spanish capital in December, and Schuster, who represented both clubs as a player, and coached Real Madrid to the Liga trophy in 2008, says victory is essential for Pep Guardiola's men.
"A Barcelona win would open up the league, and a defeat or a draw would leave the title in Madrid's hands. If they lose, it'd be a very big blow," he told AS.
Despite the gap between the two sides, Schuster went on to state that Barcelona have made better use of the resources available to them.
"Unlike Barcelona, Mourinho has used 13 or 14 players players this season. At the end of the season, there are a lot of high-tempo games. Pep was able to manage the group better."
Both sides have made numerous complaints about the quality of refereeing in La Liga across the course of the season, and Schuster says he understands the dismay at bad decisions.
"With so much pressure, it seems normal [to complain]. Errors do not help you win a title, believe you me."
Finally, the former Besiktas coach spoke about Lionel Messi, and asserted that he has never seen a player of the Argentine's quality.
"Each week he seems to grow as a footballer. I've seen many players in my time, but none like him."
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