Bwin - a sponsor of Real Madrid - tweeted the news that Casillas is heading to Porto earlier on Thursday, while also confirming that Real Madrid fans will have the opportunity to say their goodbyes to the club legend at the Bernabeu on Friday afternoon.
The Telegraph claim that Casillas’s exit has subsequently opened the door for De Gea to move to Real Madrid.
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It was reported on Wednesday by Spanish broadsheet El Confidential that Manchester United have agreed to sell the Spanish goalkeeper to Real Madrid for €40 million (£28.6 million).
De Gea, who arrived back in Manchester from Madrid for pre-season training in the early hours of Monday morning, is simply waiting for this current employers to give the green light.
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David de Gea deal closer
Casillas’s imminent exit suggests the De Gea transfer is now extremely close - and the 24-year-old is unlikely to travel with Manchester United to America on Monday.
Before agreeing to join Real Madrid, De Gea wanted assurances over first-team football at the Bernabeu. He was not prepared to join the Spanish giants in order to play second-fiddle to the World Cup winner.
Casillas, meanwhile, was not prepared to drop back down to the bench after winning his place back last season. The 33-year-old initially looked set to stay at the Bernabeu and fight for his place, but that is no longer the case.
Footage recorded in Spain last month - during the international break - indicated that the relationship between Casillas and De Gea is, at best, frosty. Although the former told reporters he would welcome the Manchester United keeper to the Bernabeu this summer.
Iker Casillas deserved more respect from fans
That Casillas has endured such a tough time at Real Madrid over the past couple of years - including long periods on the subs bench and being jeered by his own supporters - suggests the potential arrival of De Gea may have been the final straw.
He has made the difficult decision to end his 25-year association with Real Madrid to an end in order to resurrect his career with Porto.
De Gea, who was born and raised in Madrid but began his career with rivals Atletico, is the ideal replacement for the ten-time European champions.
But replacing the iconic Casillas will not be a straight-forward task for De Gea, who will be under pressure to hit the ground running at the Bernabeu.
Real Madrid fans: how do you feel about the manner in which Iker Casillas is leaving the club? Should fans have shown him more respect over the past couple of seasons? Have your say in the comments below!