Lambert has confirmed the Belgium forward will be out of action for up to six weeks with the injury he sustained during Saturday's win at Norwich.
It will keep Benteke out until November, a period that includes Belgium's final World Cup qualifiers.
However, when the the former Genk man first went down, the worry was he would be out of action for much longer.
"The medical people give you the best and worst-case scenario," said Lambert.
"You are never quite sure until you hear from the professor.
"Thankfully it is not too serious. Maybe four, five, six weeks or so."
Lambert was speaking after his side's 4-0 Capital One Cup drubbing by Tottenham at Villa Park.
The Scot felt the eventual scoreline, following two goals from Jermain Defoe in addition to efforts from Paulinho and Nacer Chadli, was harsh on his team.
And the game hinged on a bizarre incident at the start of the second half, when Jan Vertonghen stumbled and as he was falling in the box made a grab for Villa substitute Nicklas Helenius.
The Belgian succeeded only in grabbing the striker's shorts, yanking them down to his knees just as he was about to shoot.
Helenius carried on anyway, blasting over from an acute angle.
To Lambert's disbelief, referee Jon Moss opted to give a goal-kick rather than a penalty that surely would have been accompanied by Vertonghen's exit.
"It is the first time I have seen something like that happen," said the Villa chief.
"You hope you get those decisions. I don't think anybody could believe he wasn't pulled back."
Lambert wondered whether Villa would have got the spot-kick had Helenius opted to go down rather than stay on his feet and go for goal, although he did recall a similar incident at Arsenal on the opening day of the season.
"I thought that myself," he said.
"But does it matter whether he has shot or not? At Arsenal the referee waited.
"I have certainly not seen anything like that before."
Even Tottenham manager Andre Villas-Boas conceded Moss had got it wrong.
"It was a difficult decision for the referee," said Villas-Boas.
"I can understand why Paul would not be happy with that decision.
"Most likely in this situation there is no advantage to be gained and the penalty has to be whistled.
"Probably we got away with it."
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