Former Poland goalkeeper Jerzy Dudek has branded former Real Madrid teammate Iker Casillas 'the best stopper in the world'.

Casillas's form over the last 12 months has been much-criticised and he was booed by sections of the Bernabeu crowd during Tuesday's 4-3 defeat to Schalke 04 in the second leg of their last-16 Champions League clash.

Dudek, however, who represented Real Madrid between 2007 and 2011 as back-up to Casillas, has thrown his support behind the Spaniard.

"He's the undisputed number one [in the world]. I don't think there's any goalkeeper in Europe who can compare to him," Dudek told Interia.

"No team can afford to dispense with a player like Casillas. A few days ago, he said he wanted to stay at Real Madrid until he was 40. It'll be difficult because his best quality is his reflexes, and it's difficult for any player to maintain a high level [of performance] over a number of years.

"Casillas has great confidence in himself. He's a great professional and very intelligent. He's worked with the best Coaches at the highest level and always knows what to do. His success has not been achieved by chance."

Real Madrid are believed to be interested in signing Manchester United goalkeeper David de Gea this summer.