Both in Spain and England doubts have emerged over Bellingham's professionalism and commitment to his craft.
But after their Champions League win against AS Monaco, Mastantuono defended his senior teammate to reporters in the mixed zone.
“The things they're saying are harsh," said the Argentina midfielder.
"He's someone who's at Valdebebas from 9:00 in the morning until 6:00 in the evening. It's ugly when they attack your ego. I'm his friend, and he lives and breathes Real Madrid. A lot of things are unfair.”
Meanwhile, Bellingham stated yesterday: “A lot of people say a lot of things. Anyone can stand in front of a camera and say whatever they want, and people believe it. You can cry about it, you can call your lawyer, or you can enjoy it.
"I know what's going on in my life, what I give for football. I don't care about the noise from outside."