His first-half strike was his sixth goal of a season he wasn't always likely to be a part of.
Many believed the appointment of Jose Mourinho as manager would signal an end to his Old Trafford career.
Mourinho sold Mata during their time at Chelsea but the Spain international has blossomed this season.
And Redknapp believes Mata is "off the scale" and has justified Mourinho's faith with a series of eye-catching displays.
Jamie Redknapp has lauded Manchester United star Juan Mata
Manchester United 2-0 Watford: 10 key statistics from the match
Sun, February 12, 2017
Express Sport brings you 10 key statistics from Manchester United's 2-0 win over Watford at Old Trafford
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Express Sport brings you 10 key statistics from Man United's win over Watford
Juan Mata scored as Manchester United beat Watford 2-0 on Saturday
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"Juan Mata must have thought his days were numbered at Old Trafford when Jose Mourinho arrived but I am so impressed with how they have set aside their differences," he wrote in his column for the Daily Mail.
"Mourinho let Mata go at Chelsea but the manager has clearly recognised the Spaniard’s qualities.
"Mata’s footballing IQ is off the scale. He does not have the pace to burn past defenders like Anthony Martial so he plays on the right to allow him to find gaps in midfield, cut in on his left foot and shoot.
"I was not blessed with pace so I always liked to cut inside.
Juan Mata clashed with Jose Mourinho at Chelsea
"Mata’s superb display against Watford underlined his value to United — though I’m not sure about his goal celebration on Saturday."
Mourinho paid tribute to Mata for fighting for his place at the club after their Watford win.
"When I came here people thought that Mata was in trouble," Mourinho said.
"I knew that he wouldn't be in trouble because one thing is the football I wanted my Chelsea to play and another thing is the football I want my Manchester United to play and Mata has adapted to the football we want to play."