Casillas is the Madrid hero who, after being devoid of any notable competition for his spot for the best part of a decade, finds himself limited to Champions League and Copa del Rey appearances.
A winner of Champions Leagues, European Championships and a World Cup, he's now reduced to watching Diego Lopez stand between the sticks in La Liga.
It started last December when Jose Mourinho felt Casillas had begun to underachieve in his role as Madrid's No. 1 and said dropping the goalkeeper was just a "technical decision," according to Nicklas Vinde of Goal.com.
After securing a return, though, an injury against Valencia led to the signing of Lopez from Sevilla.
Since then Lopez has been good. Very good. So good, in fact, that even Carlo Ancelotti decided against the easy route of restoring Casillas to the Madrid goal.
That would have been the easy route because, at the time, it would have appeased the Madrid press and the club's fans.
The 32-year-old 'keeper has since been fighting hard to displace Lopez—his Champions League performance against Juventus was good—but, with it being a World Cup year, does he have bigger problems lying elsewhere?
Victor Valdes has been in phenomenal form for Barcelona—take this save from the weekend against Real Betis as an example—and has every right to feel he should now be considered as Spain's No. 1 stopper.
Elsewhere La Roja have a host of top-level goalkeepers playing regularly across Europe.
From Pepe Reina at Napoli, David de Gea at Manchester United through to, of course, Lopez at Madrid, Casillas has no right to expect to be in Vicente del Bosque's squad, let alone his first pick anymore.
And that's a point Del Bosque has already made in October, via Goal.com:
"We have three fantastic goalkeepers and any one of them can play. It's not a problem to decide between them. If there's a problem, it's the number of options we have."
It was more vague than his "Iker is a singular and special name in the world of Spanish football" line from August, as reported by the Associated Press.
How singular and special does he have to be to keep out the inspired Valdes at the moment?
Presuming Valdes maintains his form between now and the World Cup, if Casillas doesn't regain his regular starting sport for Madrid, it makes it really hard for Del Bosque to choose Casillas as his No. 1.
Casillas himself, in an interview with AS, highlighted the problems of not playing regularly:
Who should be Spain's No. 1?
"It isn’t easy because you play every two weeks and it’s difficult to keep your focus."
That comment almost opens the door to exclusion from Del Bosque's starting lineup, if he's not been playing, when the World Cup kicks off.
Can the Spain coach rely on someone who has himself admitted he's finding it difficult to keep his focus?
With all that considered, Casillas' exclusion from Spain's XI in Brazil is a very likely possibility the way things stand.