Two dropped points meant Chelsea were unable to regain third place from Arsenal, although they still have a game in hand, and are only a point ahead of fifth-placed Tottenham, who have played the same number of games.
Mata told chelseafc.com: "The game should have been finished.
"Something important should have happened for it to be six minutes (of added time) but it didn't."
Interim manager Rafael Benitez remains confident the club will retain their Champions League status, which was only secured last season by virtue of them winning the competition, after they finished sixth in the league.
"We can do it because we have a good team and we can win against any team," Mata added.