The Sale flanker has joined up with England in Dublin as emergency cover after number eight Nick Easter and flanker Tom Wood both suffered calf injuries.
Easter will undergo a fitness test on a "tight calf" in the morning - but England will not risk one of their senior players in a warm-up international with the World Cup just a fortnight away.
Wood has already been replaced on the bench by Stade Francais lock Tom Palmer after suffering what England manager Johnson described as a mild calf strain.
If Easter is withdrawn, then Fourie - left out of England's 30-man World Cup squad on Monday - will go straight into the team at openside flanker with James Haskell switching to number eight.
With England's resources stretched, starting lock Courtney Lawes will be called upon to provide the back-row cover.
Johnson has been forced to call on Fourie after naming only five specialist back-row forwards for the World Cup, including his injured captain Lewis Moody.
The Bath flanker has remained at England's Pennyhill Park base this weekend to work on his rehabilitation but Johnson confirmed he would fly to New Zealand with the squad on Monday.
The England medics insist Moody, who is recovering from knee ligament damage, is on course to be fit for the opening World Cup clash with Argentina on September 10.
Johnson views the Ireland game as the perfect launch-pad for England's campaign in New Zealand, which begins against Argentina on September 10.
England fielded mix-and-match teams in their warm-up matches against Wales but there is a different air to this game, there is something more at stake.
Five months ago, England had their Grand Slam dreams wrecked by Ireland at the Aviva Stadium. This is a revenge mission.
"It is a good opportunity to come back and do it," said Johnson.
"We don't have to wait two years. It is a lot of the same people involved who are coming back to have a crack.
"We want to go out and have a bloody good game of rugby tomorrow - tough, skilful, fast - to get us as battle-hardened as we can be for the first World Cup pool game.
"We have said all along we are not treating these as warm-up games."
Johnson believes a full-blown Test against Ireland a fortnight before the World Cup will not only prepare England physically but mentally for what lies ahead.
He has warned his men to focus solely on beating Ireland and block out everything else - including the very real prospect of injury.
"You don't win World Cups by thinking about World Cups," said Johnson.
"You have got to forget about the World Cup even when you are playing in it. How you handle it is a huge part of it."
That is where starting with experienced 2003 World Cup veterans Wilkinson, Steve Thompson and Mike Tindall will come into play both tomorrow and in the tournament.
"Sometimes you have to go through it. That team in 2003 had lost Grand Slams, you couldn't have written what that team had been through. It was horrible," said Johnson.
"Getting called 'chokers' and never feeling that happy with life. It kept us unfulfilled.
"We all know how big the games are when you are there but if you can separate that from what you are doing on the field, that is the best thing you can do.
"You are still playing on a rectangle of grass. You have to play with that angst and desperation all the time, it makes you play well.
"Being happy and content is not a place for elite sportsmen to be."
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