Well, if the Italian media are to be believed, and one would presume that they should be given that Mourinho apparently was talking in Italian when he leaned in close and whispered (well, yelled) into Antonio Conte's ear after the whistle, it was something about not appreciating the "humiliating" gesture of Conte asking the Chelsea fans to make some at the end of the game.
"You don't celebrate like that at 4-0, you can do it at 1-0, otherwise it's humiliating for us," said apparently the man who's never banged the badge on his chest, ran down like a maniac on the touchline, or performed an epic knee-slide to ruin several thousand dollars worth of high-grade Italian suiting material.
But that's boring. And petty. And a bit humiliating in and of itself.
Here are a few way better answers, courtesy of the Internet.
Jose Mourinho speaking into Antonio Conte's ear at length, perhaps asking how to consistently get best out of Paul Pogba.— Sport Witness (@Sport_Witness) October 23, 2016
One of these days, Pogba’s going to be amazing in the Premier League. Today was not that day.
Not a good day for Chris Smalling, and now Bailly’s going to be out for a while, too.
@BBCMOTD "If you need a translator in a few week's time, I'll be available."— Jez a Bell (@Tsarcastic_) October 23, 2016
Best Chelsea chant of the day: “You’re getting sacked in the morning!”
Roman Abramovich, is not my friend. He’s just an owner, who claimed that I was The One...
It’s good thing Conte never ever sits down!
But the best of the best, the winner, with the perfect timely pop culture reference?