After fracturing a metacarpal on his left hand in January, Spain's No. 1 has been forced to play second fiddle to Jose Mourinho's preferred keeper, Diego Lopez. Understandably, this has led to plenty of press speculation regarding his future.
Casillas has always said he would like to spend the remainder of his career in Madrid, and the man who is not selecting him looks likely to depart this summer, but under current circumstances a move elsewhere cannot be ruled out.
If he leaves—and it is a big "if"—here are five clubs that may welcome the heavily decorated goalkeeper...
With the likes of Jens Lehmann and David Seaman guarding the sticks over the years, Arsenal are used to having a world-class goalkeeper in their roster.
Polish international Wojciech Szczesny has been cast aside since March's loss to Tottenham, and his fellow countryman Lukasz Fabianski looks an unlikely full-time No. 1. This has fueled speculation that Arsene Wenger has set his sights elsewhere.
According to The Metro, Arsenal would like Casillas to be the man who saves them from Per Mertesacker's defensive errors next season.
Yet the speculative report also states that Wenger is interested in Michael Vorm and Pepe Reina. And even though an abnormal summer of spending has been promised at the Emirates, the approximate €20 million fee that Casillas would command might be too high for the famously frugal North Londoners.
Despite eight seasons of service, Pepe Reina's future at Anfield is unclear, as Brendan Rogers has apparently been "unsure" of the Spaniard since he took over (via The Daily Mail).
With the goalkeeping merry-go-round linking Reina to Barcelona—whose first-choice Victor Valdes has decided not to renew his contract next season—Liverpool are understood to have entered the race for Casillas' signature.
Spanish publications AS and La Opinion (sources both in Spanish) have both suggested Liverpool are set to join Arsenal in the pursuit for Casillas. If push came to shove, the Reds would probably be more inclined to match his rumored £100,000 weekly wage than The Gunners.
Incidentally, Iker probably does not have good memories of Anfield; the only time he played there was a 4-0 loss in 2009, and Real Madrid have never taken a point from Liverpool.
Iker Casillas made the squad for Madrid's most recent trip to Old Trafford, but didn't even make the subs bench for the game. If rumors are to be believed, he may get a better taste of Manchester as he is on Sir Alex Ferguson's radar as a potential replacement for the inconsistent David De Gea.
Furthermore, following his impressive display at the Bernabeu, De Gea was linked with a move to Madrid as a replacement for Casillas.
So, could the Spanish shot-stoppers simply replace one another? Perhaps not, if Sir Alex Ferguson's mood following the controversial match at Old Trafford is to be considered. According to the Spanish sports newspaper Marca, Fergie heaped praise on Diego Lopez, insinuating that he made saves that Casillas would not have been able to.
Paris Saint-Germain may have conceded the fewest goals in Ligue 1 with Salvatore Sirigu guarding the net this season, but the Qatari-owned side's relentless quest for a team of Galacticos implies they would jump at the chance of replacing Italy's third-choice keeper with a "box office" name like Casillas.
The five-time FIFPro World XI goalkeeper has been linked to PSG in the past, with the nouveau riche side apparently willing to part with up to €35 million to acquire his services.
Of course, if PSG succeed in tempting Jose Mourinho to Paris, it's safe to say Iker wouldn't need to buy a French phrasebook.
The first signs of the fractious relationship between Mourinho and Casillas came shortly before the winter break, when he was dropped for the trip to Malaga that Madrid ended up losing 3-2.
Could Casillas make the 300-mile trip south once again to join Los Boquerones? With their Qatari owners, they are probably the only club outside of Barcelona who could afford to pay his wages and transfer fee.
However, with Sheikh Abdullah Al Thani's curious relationship with his club—and apparent reticence to invest or clear debts that have resulted in a European competition ban—it seems unlikely that the Andalusian side would splash €20 million on a 31-year-old goalkeeper.