The 21-year-old 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 - but 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. 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 fall 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."