The Belgian international scored an injury-time winner to earn Villa a quarter-final spot for the third time in four seasons.
Benteke and Gabriel Agbonlahor had given Villa a 2-0 lead before Robins substitute Miles Storey's brace looked to have forced the game into extra-time before Benteke struck for the second time. Lambert said: "Benteke has had a quality start (at Villa) but his game wasn't up to my standards."
He added: "His two goals were world-class finishes but you set yourself a standard and we've got to keep him up to that standard. Why did he fell short? I don't know. But he has been brilliant since he came here and I think he will get better. He is only 21 and has been a pleasure to work with. He is going to be a big player and plays in a brilliant Belgium team."
Lambert felt his side were guilty of complacency after opening a two goal lead.
He said: "At 2-0 we were really comfortable and doing well and could have got another goal just before half-time. You become complacent and credit to Swindon for trying to play the right way but we deserved to win this game. I know how hard it is coming here and you've got to earn the right to win. It is not all glamour. It is a big win for us."
Lambert again left out record signing Darren Bent but his line-up indicated how seriously he is taking the competition.
He said: "You've got to take the competition seriously. There were 3,500 Villa fans here and they pay a lot of money to come and watch us. It is important we try and progress and see how far we can go."
Swindon boss Paolo Di Canio defended his actions at the end of the game when he appeared to gesture to Villa fans that their side would be relegated from the Barclays Premier League.
He said: "No, that was not the case. I was saying to my fans that they had been brilliant and their support was so good that they were up there and Villa were below them. To see the County Ground full of emotion when we scored the second goal was amazing. I was saying it was my fans are the top and they are underneath you."