The giant Belgian striker was signed for £7m from Genk in the summer and adapted quickly to life in the Premier League - netting 15 goals to keep the Villans in the hunt for survival.
Gregory, who managed Villa between 1998 and 2002, believes any criticism of Lambert's reign is unfounded and that the Scot should be given more time to prove himself.
He said: "Purely signing Benteke is enough to keep Paul Lambert in work for another two years. It was a stroke of genius.
"Whether or not Villa are able to hang onto him in the summer remains to be seen but this guy is a real find. It amazes me the likes of Arsenal have missed someone of that calibre. He scored the winning goal against QPR and seems a complete spin at £7m.
"This year, Paul Lambert was given a very limited budget. The club weren't prepared to spend money willy-nilly because it didn't seem to be working.
"Instead, Paul has invested in youth. He lost one or two of his more experienced players through injury or bad form and has had to put out inexperienced teams week after week.
"They have not been particularly good defensively but I always felt they could nick something because they have good strikers with a lot of pace.
"If Paul can manage to negotiate this season and keep the club in the Premier League, you will see a much improved Aston Villa squad next year."