Leicester are at the top of the Premier League table at the moment with 28 points from 13 matches. United are currently second in the standings, just a point behind the Foxes.
Claudio Ranieri’s side will host the Red Devils at the King Power Stadium next week, and Mata is fully aware of the challenge they will pose.
“Our next rival in the Premier League, as I said before, is Leicester,” the Spain international wrote on Kicca.com. “Ranieri’s team is having a remarkable season and the proof is that they are top of the League.
“In particular Vardy (who has equalled Van Nistelrooy's record of scoring in ten consecutive games) and Mahrez are outstanding, playing at an exceptional level... We will have to be very careful with them.”
Leicester have been the surprise package of the season so far, and indeed Vardy and Mahrez have been superb.
England international striker Jamie Vardy has scored 13 goals and created 16 chances in 13 Premier League appearances so far this campaign.
The 28-year-old has a pass accuracy of 66% and a shot accuracy of 61%, and has won 42 headers, 13 dribbles and 15 tackles.
Winger Mahrez has scored seven goals and created 29 chances in 12 Premier League games this season.
The Algeria international has a pass accuracy of 75% and a shot accuracy of 58%, and has won 12 headers, 44 dribbles and 13 tackles.