The 21-year-old striker scored twice at the County Ground on Tuesday as Villa defeated Swindon 3-2 to reach the quarter-finals of the League Cup.
"Benteke has had a quality start [at Villa] but his game wasn't up to my standards," Lambert is quoted as saying by Sky Sports News.
"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."
The League Cup quarter-finals draw will be made on Wednesday.