Mourinho, who is rumored to be on his way out of the club at the end of the season, will not have endeared himself to the supporters, players or Madrid media with such a radical move.
It was the decision of a man clearly under pressure after what has been a calamitous Liga season to date. The defeat left Madrid 16 points behind runaway leaders Barcelona and seven off city rivals Atletico.
It was also the decision of a man who cares little what others think. Dropping Casillas for the little known Adan was a decision Mourinho knew would set tongues wagging in Madrid.
Casillas is as close to undroppable as any player in world soccer - and the fact he is a goalkeeper means Mourinho was unable to claim he was being 'rested'.
Mourinho's relationship with several of the Spanish players is long said to have been strained, and one of those players whom he has fallen out with in the past, Sergio Ramos, admitted: "It surprised us, Iker is our captain."
Mourinho claimed the choice of goalkeeper "had no influence in the result" but this was the first time that Madrid had conceded three goals in the league against anyone other than Barcelona since April 2011.
If you are thinking about taking a trip to Spain to watch Real Madrid, or any of the other Spanish teams, then it is definitely worth a visit to Spain TicketáBureau for great ticket prices.