Villa are teetering above the relegation places and he puts their poor form this season - made worse by a 3-1 loss to League Two Bradford on Tuesday - down to fielding an inexperienced side.
Staunton, who played almost 300 games over two spells at Villa, said: "The game against Southampton, after getting stuffed 4-0 down there earlier in the season, is massive, it's crucial. This is the biggest game the club has faced for a very long time. One of the biggest they've played.
"If you look at the last three years it's been going this way for a long time. You can see the lack of experience in the squad but Paul Lambert's had to play the youngsters - there's no choice in the matter that's where the squad's at."