The 21-year-old and Gabriel Agbonlahor both scored a nine-minute spell just before half-time, but Swindon fought back courtesy of a brace from Miles Storey.
However, Lambert’s men managed to avoid extra-time and possible penalties as their Belgian striker netted in the 90th minute to book Villa’s place in the fifth round.
“Benteke has had a quality start at Villa but his game wasn’t up to my standards,” said Lambert.
“His two goals were world-class finishes but you set yourself a standard and we’ve got to keep him up to that. Why did he fell short? I don’t know.
“But he has been brilliant since he came here and he will get better. He is only 21 and has been a pleasure to work with. He is going to be a big player.”
Villa are currently 17th in the Premier League and without a win since the 15 September, and Lambert believes their Capital One Cup victory could prove vital in improving their league form.
“At 2-0 we were really comfortable and doing well and could have got another goal just before half-time,” he added.
“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.”
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