Mata has fond memories of this week's opponents Benfica, having defeated the Portuguese team while at Chelsea during the club's Champions League-winning campaign in 2012 and a year later in the final of the Europa League.
However, the midfielder has insisted that he will not be complacent at the Estadio da Luz in two days' time.
In his weekly blog, Mata wrote: "Despite these good memories I don't want to be overconfident. They are a team with very good players and a big supporting crowd.
"They need the points and we will have to be at our best in order to get a positive result. At the end this is the Champions League and you have to beat great teams if you want to reach far."
United are unbeaten in 11 games in all competitions this season.