United spent in excess of £37m in luring the 25-year-old to Old Trafford from Chelsea in January, and the Scot believes that the hefty transfer fee is proving money well spent.
"I think Juan Mata's played really well and I think he's made us better," Moyes told reporters.
"He hasn't had the goals in the early games but I think he played fine. Recently he's started getting the goals and, more importantly, the assists as well.
"He's always got himself a good tally of goals and we needed that. He's coming up with the goods now."
The Red Devils moved up to sixth in the table following the win over the Magpies.