The forward has been in fine form so far this season, netting for the Three Lions in their victory over Slovakia on Monday.
He was also on target in the 2-0 win for United against Leicester before the international break.
And Mata knew Rashford was a special talent the first time he trained with him.
“I’m not surprised,” the Spaniard said on talkSPORT.
“Since the first moment we saw him in training, we thought he was something special.
“And he’s making it real. He’s scoring for England, for us [Manchester United] and he has been a really good player and a real threat for opposition defenders.
“I think he can be even bigger. He has a great mentality and great qualities to become a big, big player.”
Rashford will hope to continue his good form for United when they travel to Stoke on Saturday.
The Red Devils are currently top of the table after winning all three of their Premier League matches to date.
Rashford will also have his sights set on being included in England’s World Cup squad next summer should they qualify.
