The 36-year-old centre makes a 12th Test match appearance against the Wallabies in as many years after answering England head coach Brian Ashton's SOS to replace an injured Andy Farrell.
Catt's 73rd cap - he moves above Will Carling on the all-time England list - coincides with the reigning world champions fighting for their lives to stay afloat in a tournament after South Africa seemingly sunk them without trace three weeks ago.
A spirited recovery that produced bonus-point victories over Samoa and Tonga has at least given England a fighting chance of dumping Australia from World Cup contention for the third time in four tournaments.
But the Wallabies start as red-hot favourites, even though Saturday's local forecast for Stade Velodrome suggests an English-style downpour following a week of temperatures in the high 20s.
Catt, axed after the Springboks shambles, has leapfrogged Olly Barkley for England's number 12 shirt, with a decision on Barkley's fitness being made during the next 24 hours.
If the Bath back fails to recover from a dead leg suffered during training on Wednesday, his place among the substitutes will be taken by Newcastle's Toby Flood.
For Catt though, it is an opportunity to play alongside Jonny Wilkinson for the first time since England's 2003 World Cup semi-final victory over France in Sydney.
He said: "It has been pretty disappointing, personally, over the past four weeks, so this is another chance for me to get out there and perform.
"I have always said never say never, and two days in rugby union is a long time, going from being number three to number one.
"Things happen, and unfortunately 'Faz' has picked up an injury and Olly Barkley has a knock.
"I haven't played with Wilko for four years, so I am looking forward to being next to him on the pitch.
"A lot of things have happened in the past four years, so we will just see how the game progresses. We need to work as a pairing in terms of our decision-making."
The kicking games of both players are likely to be employed far more than any attacking adventure, judging by the power-packed forward unit Ashton has selected.
And if England do progress towards a Paris semi-final against New Zealand or France on Saturday week, the 30,000 red rose supporters expected to throng Marseille cannot expect to enjoy a try-fest.