Man City kicked off their European venture with a thumping 6-3 victory against RB Leipzig.
Grealish was among the scorers at the Etihad, bagging a stunning strike on his Champions League debut.
But Guardiola was NOT pleased with his defensive efforts and told him about it in full Pep mode.
He wasn't the only Man City star on the end of an ear-bashing as Mahrez also received a lecture.
A video shows an animated Guardiola talking Grealish and Mahrez through defensive work.
Jack Grealish: :soccer::a:️- Football on BT Sport (@btsportfootball) September 15, 2021
Riyad Mahrez: :soccer:
It doesn't matter how well you're playing, if you don't do your defensive duties Pep Guardiola will tell you all about it :triumph:#UCL pic.twitter.com/f9ZQFxaq4w
Asked why he was unhappy with the pair of forwards, the Spaniard replied: "We spoke in half-time about the way we had to do it and they didn't do it. But it happens.
"They [Leipzig] are so good, they make four players in the build-up to attract you, and after they find you behind the holding midfielders, where there are a lot of players, when the ball goes outside put it inside.
"And after they are so fast to attack in behind. So, it's not absolutely easy to control it."
On his conversation with Guardiola, Grealish added: "That was just defensive work.
"I won't go too much into it because he might want the same tactics for the weekend!
"That's what the manager's like, he's always wanting more defensively and offensively.
"And obviously he's someone that I'm going to listen to after everything he's done in the game. So obviously he's just giving out useful information."
Featured Image Credit: BT Sport
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