Villa could even have completed their first double over United for 55 years after the first half sending-off of Nani for a two-footed challenge on skipper Stiliyan Petrov.
But Cuellar is adamant Villa should not be overawed when facing Sir Alex Ferguson's side at Wembley on February 28.
Cuellar said: "To get four points out of Manchester United this season gives us a lot of confidence.
"It is very good for the team's belief because we have showed we can get points - and win - against the best team in this league, one of the best teams in the world.
"Will we be favourites for the final? It will be hard but we can do it. We have the confidence from these two league games.
"The final will be different but we believe we can win the game and we won't be scared of United.
"It will be important if we can win a trophy because the team is getting better and improving every year. We have some really excellent players at this club.
"It would be good for the fans and for the club to lift some silverware."
Cuellar knows Villa will have to keep close tabs on Wayne Rooney after the United striker produced another excellent performance as the lone striker.
And he feels England will have an excellent chance of winning the World Cup this summer if he sustains his current level of performance.
Cuellar said: "Rooney is playing very well. He is on fire now. He is scoring a lot of goals. He will cause anyone lots of problems
"If Rooney continues to play like that, England can win the World Cup.
"England are one of the best national teams in the world and they have players who are scoring goals but Rooney is really on the ball."
Cuellar admits Villa found it hard to break down United after Nani's dismissal.
He said: "Always, when you are playing 10 men, you have the obligation to try and win the game.
"But, sometimes against ten men, it is hard because they are more concentrated, and defend more, and you don't have too many spaces to try and exploit."
Villa boss Martin O'Neill will be sweating on the fitness of Petrov who has been in inspired form this season.
He has already been defying an Achilles problem to lead Villa's push for a European spot before suffering an ankle setback via Nani's challenge.
The Bulgarian international is rated a doubtful starter for Sunday's FA Cup fifth-round clash with Crystal Palace at Selhurst Park.