St Ledger goes into his World Cup fixture against Italy well aware victory would send them into Wednesday's clash with Montenegro still in with a chance of overhauling the leaders.
St Ledger was not in the squad for the 1-1 draw between the sides in Bari on April 1.
St Ledger: "I would imagine that, when the fixtures came out, this would be the game everyone was looking at and, from a footballer's point of view, it is probably the same.
"You have to take each game one step at a time but, now the game is here, it is massive.
"I wasn't in the squad to go to Italy, which was obviously disappointing at the time, but it is going to be strange walking out with the likes of (Gennaro) Gattuso and people like that.
"Only a couple of years ago, I was playing against Bradford in front of 2,000 people and their dogs. It's a major difference but something I would like to taste more of.
"It's massive. It's the biggest game of my career to date. We are playing one of the great football nations, not just now but in terms of history as well.
"If you look at the importance of the game, realistically, we are probably looking at the play-offs and that's going to be tough for us.
"But we have got nothing much to lose. We can go out there and try to beat the Italians."