Having watched Liverpool beat Burnley earlier in the day, Villa went into their fixture at Old Trafford knowing that three points would take them joint-top of the Premier League with the Merseyside giants.
At half time, everything was going to plan, Villa having taken a 2-0 lead courtesy of goals from John McGinn and Leander Dendoncker.
However, United were much improved and clinical in the second half as two strikes from Alejandro Garnacho and Rasmus Hojlund's first-ever Premier League goal earned the home side a deserved three points.
After collecting just one point from a possible six over the past four days, Villa remain third and are at risk of soon being overtaken by Tottenham Hotspur and Manchester City.
Speaking to Amazon Prime after the game, Emery suggested that his previous view that Villa were not genuine title contenders was now justified.
The Spaniard said: "Two weeks ago I was asked if we were contenders to the title and said no, and was thinking the same as I am now. Why? Because we are playing several top teams that are contenders more than we are. Of course we are being consistent and better than a lot of teams so far this season.
"Today we missed an opportunity to have that consistency against one of the top ten teams. This can happen. The reaction after the first goal conceded was good, we even then created a few very good chances, but that is football.
When quizzed on how he wanted his players to react, he added: "Move on, be demanding. I told the players that the first part of the season was fantastic but I want more. If they want more we have to work hard to get it."
Defeat at Old Trafford brought to an end a 10-match undefeated streak in all competitions, one that had lasted since November 5.
Villa are back in action on Saturday when they play host to Burnley at Villa Park, opponents who sit in 19th position in the standings.