The Villans' coach felt that the 21-year-old's overall performance wasn't up to the standard he expects despite two "world-class" finishes that sealed their win against Swindon
The 21-year-old took his goal tally to four from eight games for the Villans and he opened and closed the scoring in the win but Lambert feels as though his overall performance was below par.
"Benteke has had a quality start at Villa but his game wasn't up to my standards," Lambert told reporters.
"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 fall 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."
Despite progressing to the quarter-finals, Lambert felt that complacency replaced consistency after Benteke opened the scoring and Gabriel Agbonlahor made it 2-0 at half-time, with Miles Shorey levelling the scores with with two late goals for Paulo Di Canio's side.
"At 2-0 we were really comfortable and doing well and could have got another goal just before half-time," he continued.
"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."