Iker Casillas' Real Madrid career stagnated under Jose Mourinho due to his active attempts to unite his teammates with their Barcelona counterparts on international duty, according to Vicente Del Bosque.
Under Mourinho, El Clasico encounters became frequently heated, with this culminating in the Portuguese poking a finger in the eye of Tito Vilanova during a clash in the Spanish Super Cup in 2011.
And Del Bosque has revealed how Casillas' attempts to ease tension for the good of the national team affected his club position.
He told radio station Onda Cero: "It's true that in difficult moments for the national team, when the Real-Barca matches got a little strange, he (Casillas) ... helped the team to move on and continue winning things like the (2012) European Championship.
"This did not go down well and perhaps had a detrimental effect also on Iker, he was a key figure in fostering harmony in the national team."
In the lead up to Euro 2012, Casillas allegedly had meetings with Barcelona players Xavi and Carles Puyol in order to stop the bad blood and keep the nation focused on retaining their crown.
Del Bosque suggested that this could have caused friction between the goalkeeper and current Chelsea boss Mourinho, who dropped him the following season in favour of Diego Lopez.