Jose Mourinho has never been afraid to rub his players up the wrong way.
The Portuguese manager is notorious for having a poor relationship with some of his players.
Although his methods have often got results, his ways of getting the best out of his players has not always gone down too well.
Iker Casillas is just one high profile name that has fallen out with Mourinho.
Mourinho dropped Casillas when the two were at Real Madrid, with Diego Lopez preferred in his place.
And Casillas has now come out and said he regrets not confronting Mourinho.
"He was brought in to compete with Barcelona and brought moments of great tension and that Madridismo that I mentioned that I don't like came out, there was a limit that he went to with me that I didn't want to go to," Casillas told Universo Valdano channel Vamos, per Goal.
"The third year wasn't all good, although we won La Liga and it seems that there were many people whose relationship was shaken.
"It wasn't good for him, nor for me nor for the club. I think if it happened all over again, I would have taken the bull by the horns and faced Mourinho.
"At that time, I opted to be quiet and I thought that was the way to honour the values of Real Madrid.
"Nobody has wanted to talk about Mourinho to this day and I think it is maybe better to let it be and let it go."
That was always going to entice a response from Mourinho, and he's now done so in brilliant fashion.
In an interview with Record, Mourinho replied: "The interview is that of someone who is at the end of his career.
"When it comes to my person, when he says that he never confronted me that is not true, he confronted me and did it in the way that there is no one to do better than him: hidden."
Ouch. That is just vintage Mourinho. He has truly destroyed Casillas.
It'll be interesting to see if the Spanish goalkeeper has any response to Mourinho.
But, after the Manchester United manager's reply, it's probably best that Casillas keeps quiet.