Manchester City demolished RB Leipzig 6-3 at the Etihad Stadium in their first Champions League fixture on Wednesday night.
Both Grealish and Mahrez were on the scoresheet to make sure they record a first Champions League group stage win.
Guardiola was seen screaming in the faces of his two offensive players as they make their way to the dressing room during the half-time break.
And when quizzed on his reaction towards the two players, the Catalan coach disclosed he was not happy with their defensive efforts in the game.
"We spoke in half-time about the way we have to do it and they didn't do it. Fights happen.
"They are so good, they make four players in the build up to attract you behind the holding midfielders where there aren't a lot of players, where the ball goes outside and they are so fast when they attack in behind.
"It's not easy to control it but when you regain that ball there are many players in front of the ball and in the transition to make one or two passes in behind you have a chance to attack.”
Pep Guardiola since winning the Champions League trophy with Barcelona in the 2008-2009 season has never won the accolade despite having a success spell at German side Bayern Munich before joining Manchester City.
The 50-year-old tactician came close in winning the title last season but were beaten by Chelsea 1-0 in the final.
