Despite the team's lowly position this term, Fletcher believes that the capture of Mata will stand them in good stead for next season.
"It's been a disappointing season but he has come in and been fantastic," Fletcher told the Daily Star.
"Juan's a classic No 10, controlling games with an eye for the killer pass or he can slow it down with two or three touches.
"He gives you a bit of everything, composure, intelligence, assists and goals. Even off the ball, although he may not be making tackles, he's here, there and everywhere and reads the game so well."
Mata is cup-tied for United's Champions League quarter-final second leg against Bayern Munich on Wednesday.