Definitely Aston Villa is going to win Manchester United on Sunday at the Villa Park. Aston Villa are in good form this season they have been consistent and no team has beaten them this season in the English premier league as they are still playing unbeaten where as Manchester United has not got there form this season as they have lost 3 already in the English premier league at of 6 game played.
Just as you said they have more confidence now after defeating Bayern Munich in the UEFA Champions league, I don't think anybody will bet in favour of Manchester United to win Aston Villa at the Villa Park on Sunday as Aston Villa is favorite to win Manchester United due to there current form and wonderful performance against Bayern Munich in there UEFA Champions league game.
Aston Villa statistics are better than Manchester United, so far Unai Emery squad has managed to collect 13 points. The results were obtained from 4 wins, 1 draw and 1 loss, they are ranked fifth in the standings. Manchester United are 6 points behind Aston Villa, and they are currently ranked thirteenth. Therefore, it makes sense to place Aston Villa as the favorite team in the match later, especially since they will play in front of their own supporters.
Both teams returned from European competition with different results, where Aston Villa surprisingly managed to beat Bayern Munich, which is a heavyweight team. Meanwhile, Manchester United again failed to get full points in a row. As a result, these results will more or less affect the mentality of the players, and the absence of Bruno Fernandes due to a red card received in the previous match against Tottenham, will benefit Aston Villa.