The Barcelona playmaker's father has revealed that the Real Madrid coach had issues with the duo's good rapport, and hailed their role in Spain's Euro 2012 success
The two have been friends ever since they were teenagers, but their strong bond was put to the test during the heated encounters between Barca and Madrid over the past few years.
Casillas and Xavi managed to put any conflicts aside off the field, and although the Barcelona playmaker's father has cited the duo's behaviour as one of the reasons for Spain's success at international level, Mourinho was unhappy that his skipper remained friends with a rival player.
"Iker got into a lot of trouble with Mourinho over his friendship with Xavi. These problems became public last season and everybody knows about it. Xavi and Iker never had any intention to end their friendship," the Barcelona star's father said to Com Radio.
"If it weren't for Xavi, Casillas and Vicente del Bosque, Spain would not have come this far. The Clasicos led to difficult moments. Relationships between players who used to be friends were about to go sour.
"There were issues with [Alvaro] Arbeloa, Xabi Alonso and Sergio Ramos among others. The behaviour of some players wasn't appropriate. Luckily Iker and Xavi then got involved."
Despite reports of a divided squad in the lead up to Spain's Euro 2012 campaign, La Roja successfully defended their European title this summer by beating Italy 4-0 in the final.