Messi has been at the pinnacle of world football for the past decade now.
Teams all across Europe have tried and failed to prevent the Argentina international impacting the game.
And Mourinho found out the hard way in his first El Clasico as Real boss back in 2010, with Messi running the show and Barcelona ending up 5-0 winners.
However, the Portuguese coach quickly hatched a plan to halt Messi and that involved Alonso personally marking the 30-year-old out of the game.
And the former Spain international has revealed how Mourinho “sacrificed” a lot of make sure Messi was unable to affect the game.
“Messi has done a lot of damage to me, Mourinho and Sergio Ramos talked about him a lot,” Alonso told Ecos del Balne.
“We only started to control Messi when I was marking him.
“We sacrificed positions for me to mark him, but once we controlled Messi the matches were even.”
He added: ”The famous 5-0 made us change a lot of things.
“At that time we weren’t ready to compete, a year later we were.
“Barcelona were dominant, but by the end of 2011 we knew we could strike against anybody in four moves.”