The Spaniard played under Mourinho at Real Madrid, winning La Liga in the 2011-12 season and won two consecutive Bundesliga titles as he moved to Bayern Munich a year after Guardiola first took to the helm.
But having achieved success under both, it was a game where the pair clashed that Alonso perceives as his lowest moment in his 17 year professional career.
Mourinho and Guardiola faced up for the first time in Spanish football with the former succumbing to an embarrassing 5-0 defeat at the Nou Camp.
Though Alonso, speaking to Sky Sports, in hindsight, states the torturing match, where he started in the centre, acted as a drive to establish improvement at the Santiago Bernabeu.
"I have memories of the first Clasico between Pep and Jose, it was all about them,” the 34-year-old maestro told Sky Sports. “Because they are big personalities, and are so attractive to the fans, so there were big expectations.
"The first game was 5-0 to Barcelona. It was my worst experience on a football pitch. After 20 minutes I wanted the game to be over.
"It was a big setback for all of us, and Jose as well, but at that moment we were not at the highest level we could be. We kept going.
"It took about a year-and-a-half for Real to get to where we wanted, and from then we were level with Barca I think.”
From the first whistle to the last, arguably the best club side ever played Los Blancos off the park. Not many managers can say their team have done that.
Goals from Xavi Hernandez and Pedro Rodriguez put the Catalans 2-0 ahead and Alonso and all his teammates fell apart.
The managers renew their rivalry in the Premier League on Saturday. And both Manchester United and Manchester City boast 100% records after three matches.
Jose Mourinho will be hoping his players do not record a similar experience as Alonso's six years ago.