The former Germany midfielder has warned the Catalan club that they must win Saturday's Clasico showdown if they are to stand any chance of winning the Spanish 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 that 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.
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"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," he observed. "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 the German added that 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," said Schuster.
Finally, the former Besiktas coach spoke about Barca superstar Lionel Messi, asserting 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," he marveled.
Goal.com will be hosting a LIVE Twitter Q&A on Friday at 14:00BST as Goal.com Spain Editor Martin del Palacio Langer and Goal.com International Features Editor Carlo Garganese debate whether Lionel Messi and Cristiano Ronaldo are the two greatest players of all time. Throughout the LIVE we want to hear your thoughts and opinions on the subject, while we will also be putting your questions to Martin and Carlo as we look to bring the debate to an end.