Manchester City boast strength in depth throughout their squad, with two players battling it out for one place in some positions such as Pablo Zabaleta and Micah Richards at right-back.
Except in the goalkeeping department, where Joe Hart has ruled the roost since he got the nod from Roberto Mancini ahead of Shay Given at the start of the 2010-11 season.
And for most of that time, it's not been a problem until some recent wobbles have raised questions as to whether Hart needs pushing by a deputy of proven quality and experience.
Given lasted one more year after Hart became their No. 1, but were Hart's excellent displays that season down to the fact that he had a classy and battle-hardened performer on the bench and pushing him in training?
That was the season Hart established himself as England's first-choice custodian, and the following summer City replaced the departed Given with Costel Pantilimon. The lanky Romanian has not let anyone down when given the nod over Hart in cup ties and was unlucky to be replaced the FA Cup Final last May.
It would be a major surprise if Manuel Pellegrini opted for Pantilimon, but what if Casillas was to arrive at the club in January?
"We are not going to sign Iker right now but we all know what a great keeper he is, and the importance he has in a team. At City there are great keepers, but Iker would be a great reinforcement."
Those were the words of City's Jesus Navas, from Spanish paper AS and quoted by the Daily Express recently.
With the Real stopper seemingly out of the picture at the Bernabeu, it was inevitable his name would crop up during the recent transfer window.
City, as well as Arsenal and Paris St-Germain, have been linked with the 32-year-old, and if he were to go to the Blues it would mark a major shift in the dynamic. It's hard to see someone of Casillas' caliber going to be a No. 2 and equally hard to see Hart liking it if he was supplanted as first choice between the sticks.
If, and it is a big if, Casillas were to arrive it might just spur Hart on to new heights to retain the number one jersey. It could be a win-win situation for City, if not for the keeper who sits on the bench.
Signing a world-class performer like Casillas would hardly be a bad move for City, but it looks like a remote possibility with a distinct whiff of paper talk right now. Hart's performances between now and the opening of the January transfer window may determine whether it becomes more realistic.