The Spaniard, who swapped Chelsea for Old Trafford in January, netted twice as United cruised to a 4-0 victory over Newcastle on Saturday.
Despite their win at St James' Park at the weekend, defending champions United remain a lowly sixth in the Premier League standings.
However, Fletcher believes the addition of playmaker Mata stands them in great stead for next season, even if David Moyes will be without his star signing on Wednesday as they travel to Germany for a Champions League quarter-final second leg against holders Bayern Munich.
"There is so much quality in that dressing room and players are going out there with a point to prove," said Fletcher in the Daily Star.
"Of course, it's a shame he can't play against Bayern, but we always knew that.
"It's been a disappointing season but he has come in and been fantastic."
Fletcher is convinced the £37.1million signing has a bright future at Old Trafford and believes the 25-year-old has the credentials to dictate proceedings for United.
"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 added.
"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 has become renowned for his high work-rate and Fletcher insists he is continually improving, giving United renewed hope for next season.
"He covers tremendous distance and if your see his stats, they're always one of the highest in the team," the Scotland international said.
"He's getting better and better, so it's a mouth-watering prospect for next season when we'll be looking to push on.
"And if we can produce the form we showed against Newcastle, then we can really get back to challenging for the title again."