Spanish midfielder Mata joined United for £37m in the January transfer window and has since scored three goals in 11 Premier League games for the club.
"I think it's gone a little bit amiss but Mata has been terrific in his play," Moyes told reporters, according to Goal.com.
"One thing I will say is it's been missed how well Mata has done. He's started to assist goals and score goals and has struck up an understanding with Wayne [Rooney] and with who he's played with.
"Juan has been a little bit overlooked. I could nearly say you (the media) don't want to talk about anything so positive, but he's been really good."
United currently find themselves in seventh position in the Premier League table.