The Reds twice gave away two-goal leads as they dropped points in the race for the Premier League title.
"There is anger from the players, of course, because we wanted to win," Nani told MUTV.
"We're disappointed to lose four goals, but that's football. It can happen sometimes.
"We have to keep believing in our qualities because we still have a great oppportunity to win the title."
Manchester City closed the gap at the top of the table to three points following their 2-0 win at Wolverhampton Wanderers. United and City meet at the Etihad Stadium in what is being billed as a title decider on Monday, April 30.