Martial was key in Manchester United's 2-0 win over Watford yesterday.
The France striker set up Mata for the opener at Old Trafford and scored United's second with a cool finish after the break.
The 21-year-old's future at the Red Devils has been called into question in recent weeks after Jose Mourinho publicly told him to improve his performances.
Juan Mata hails Manchester United team-mate Anthony Martial
And Mata has backed "great friend" Martial to kick on in the coming weeks.
"I was very happy for him," Mata told MUTV. "There were some difficult weeks for him, but he has been training good.
"He's a great friend, he's a great player and he can give us a lot of things and now he's with confidence, he's a terrific player.”
Anthony Martial scored Manchester United's second at Old Trafford
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Martial enjoyed a stunning debut campaign at United – scoring 20 goals and providing 16 assists – but has found the back of the net just seven times this term.
United's win over Watford saw them move to within two points of second-place Tottenham, who lost 2-0 to top-four rivals Liverpool yesterday.