The Scot managed Villa between June 2012 and February 2015, before being sacked and replaced by Tim Sherwood.
Sherwood revealed in his pre-Wembley news conference that the finalists may not be able to prevent the Belgium forward from leaving the club due to a reported ??32.5m buy-out clause in the player’s contract.
Lambert signed Benteke from Genk in August 2012 for ??7m and told Sky Sports News HQ he was mystified by the clause becoming public.
He said:??”I’m surprised by the timing of it, a day before the cup final. That’s the surprising thing.??
“I fought tooth and nail to keep Christian there from day one because he was playing that well in his first year.??
“The second year he had his (Achilles) injury and there were so many teams looking at him, calling for him, and I dug my heels in because I thought it was the best thing for Aston Villa for Christian to stay.
“I always used to say to him, ‘give me one more year, one more year’ and he was great with it and I’m delighted he’s turning out to be the player I always thought he could be.”
Benteke scored 26 goals in 44 appearances for Villa in his first season and has struck 12 goals in 12 games ahead of the FA Cup final and Lambert is not surprised by the Belgian’s success in front of goal.
Lambert said: “It took him a few months to find his feet in the Premier League. I know he scored on his debut, but he then had a bit of a barren spell.
“Christian Benteke in his first year was incredible, only behind Ronaldo and Messi in the scoring charts at that time, so we always knew he could perform.
“What I’m saying now, doesn’t change my opinion of him because I know what he can do.”