The Catalan giants have been linked with a move for Mathieu during the close-season with Marc Bartra and Gerard Pique the only recognised central defenders in Luis Enrique's squad.
Club captain Carles Puyol has retired, and new coach Luis Enrique has hinted that he plans to use Javier Mascherano - who has filled in at centre-back since arriving from Liverpool in 2010 - in his favoured midfield role.
However, Valencia president Amadeo Salvo has warned Barca they will need to up their offer before he even considers selling Mathieu.
"We will not sell Mathieu if the club does not come with €20 million in cash," Silva is quoted as saying by Marca.
Former Toulouse man Mathieu played 32 La Liga games last season as Valencia finished in eighth place.
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