The 19-year-old has been tipped as a potential superstar and has frequently uploaded videos of himself freestyling with the ball onto Youtube.
But his manager claims the midfielder no longer partakes in such showboating after he offered him some strong words of advice.
Speaking to Football Italia, Gattuso said: “We've talked a lot lately. I even threatened him because he became more famous for making videos than playing, but he doesn't do that anymore because I told him I'd knock his teeth out!”
Mastour shot into the spotlight when he joining Milan at 14 and gave the world its first glimpse of his talents by keepy-ups with a ping pong ball and a cherry during an interview with Sky Italia.
Although that made him a hit with Youtubers, his continued uploads have angered his boss and could be one of the reasons behind his lack of match action.
The teen clearly possesses plenty of natural ability, but Milan clearly feel that he has not developed as they predicted and he has not made a single appearance for them this season.
And Mastour, who is out of contract next year, has done himself no favours when given an opportunity elsewhere after underwhelming loan spells in Spain with Malaga and Holland at Zwolle.
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But Gattuso has not given up on the youngster just yet and believes he has seen plenty of improvement.
The San Siro legend added: “In recent months he's improved some things in training, and we decided to let him play in the Primavera.
“The train has passed but he's not 50-years-old, he's 20-years-old [in June], and I think he has to take stock of his mistakes.
“He needs to play consistently because we can see he's lost some match sharpness.
“I've noticed a few improvements though.”
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