The England international committed his future to the Red Devils until June 2019 with a new contract said to be worth £300,000 per week.
In Rooney's first game since putting pen to paper on fresh terms, United secured a 2-0 win over Crystal Palace at Selhurst Park on Saturday.
Mata wrote in his weekly blog: "The game was special for Rooney after his recent renewal, and he was very happy.
"He deserves it - he is a model player in this club and it's a privilege to have him with us now and in the coming years."
United are now sixth in the Premier League table, 11 points adrift of fourth-placed Liverpool.