Yes, he did.
That was just my gut talking, my first reaction to the question. Obviously, football is a team sport, and it is very rare that a single player can cost any team a league trophy but, as I see it, Casillas is responsible for most of it. Real Madrid's captain has not been able to perform at the level we should expect of him, who was once the best goalkeeper in the world, or of any player who wears Real Madrid's T-shirt.
And this is, basically, why I think that Casillas costed Los Blancos the league trophy. Most of the people who defend our goalkeeper whenever Real Madrid receives a goal argue that it was a hard shot to block, and they may be right most of the times. But that is just not what makes a player deserve to play for Real Madrid. This team aims at configuring a lineup with eleven plausible contenders for the Ballon D'Or trophy. With all due respect to smaller teams, not making blunders does not suffice to play here, and this is what many people seem to be asking Iker for right now.
One of those is, obviously, Real Madrid's coach Carlo Ancelotti. He spends day after day in Valdebebas, watching his players train, and therefore has more information than I do on the topic, but I can't just understand how he decided to keep Casillas over Diego López last summer, or how he chooses Iker over Keylor Navas every weekend. At least, not from a purely footballing standpoint.
In any case, neither Diego López nor Keylor Navas (who did a fantastic job last season and in the World Cup) are Real Madrid-quality goalkeepers. If we were to ask our goalkeeper for the same output than we ask field players for, our natural candidates would be Courtois and Neuer, and perhaps De Gea. But the thing is that Barcelona and Atlético de Madrid have had clearly better goalkeepers than us this season, and so have Valencia, Bayern, Chelsea or Juventus, and competing in such conditions is very difficult.
Focusing now in La Liga, I don't recall many blunders by Casillas (although I should remember none, if things worked as they are supposed to), but Casillas has played a significant role in most of the points Los Blancos have missed throughout the season:
- Real Sociedad - Real Madrid (4-2): Real Madrid were leading by 0-2 in the first half, and Real Sociedad were able to equalize before the break via a corner kick that was poorly defended by Los Blancos and goal by Zurutuza that Casillas should have avoided. In the second half, Casillas could have done more to cut the final pass in the 3-2, and the 4-2 came again from a corner kick in which our goalkeeper was complaining to the referee while Carlos Vela was shooting.
- Real Madrid - Atlético de Madrid (1-2): Atlético's first goal came, again, from a corner kick at the near post that, with a regular goalkeeper, would have never been a problem.
- Valencia - Real Madrid (2-1): Casillas could hardly have avoided any of Valencia's goals, although it is worth mentioning that the second one came, again, from a corner kick.
- Atlético de Madrid - Real Madrid (4-0): It would be unfair to say that any player is responsible for this defeat, as the team was horrible as a whole. Still, Atlético de Madrid's first goal came from an unforgivable blunder by Casillas.
- Real Madrid - Villarreal (1-1): When Real Madrid were leading by 1-0, Gerard Moreno managed to equalize with a great shot from the top of the box that barely no goalkeeper would have been able to block.
- Athletic Bilbao - Real Madrid (1-0): Although Pepe's poor defence allowed Aduriz to get a good and well adjusted header after a cross by Mikel Rico, it was not so powerful to make me think that a very good goalkeeper would have not been able to get it.
- FC Barcelona - Real Madrid (2-1): Barcelona's first goal came from a header by Mathieu in a set piece that came in through the center of the goal, just half a meter away from Casillas' position. Their second goal, by Suárez, is a blunder by Casillas, who decided to go to the wrong post before Suárez shot.
- Real Madrid - Valencia (2-2): Valencia's first goal came from a header by Alcácer that Casillas touched but missed to block. The second one came from a header by Javi Fuego in a set piece.