The England wing received a one-match ban at a disciplinary hearing in Auckland on Monday after pleading guilty to a "dangerous high tackle" on Scotland's Chris Paterson.
The independent judicial officer, Bruce Squire QC, halved Armitage's potential ban after taking into account a number of mitigating factors.
Armitage will be available to play in the semi-finals, should England beat France in Auckland next Saturday.
England have a ready-made replacement for Armitage in the form of Mark Cueto, who missed out on facing Scotland despite scoring a quick-fire hat-trick against Romania.
Cueto had played 29 consecutive games for England before a back injury ruled him out of their World Cup victories over Argentina and Georgia.
The Sale wing admitted he "spat the dummy" after England manager Martin Johnson opted to start with Armitage on the left wing against Scotland.
Cueto felt fully fit after his appearance against Romania - but the England coaching staff disagreed.
"To play the way I did and score the hat-trick, it was disappointing to find out I wasn't playing last week," Cueto said today.
"I was 100% fit. The coaches weren't convinced I was 100%. They know me pretty well and they thought I was just lacking a little edge in that game.
"They thought I needed another week's training, get that under my belt and be OK.
"You try and convince them to change their minds and whatever else but the decision was made.
"It was something I had to deal with. I spat my dummy out on the day I was told but it is a squad effort.
"There are disappointments for everyone along the way and you can't let that get in the way of what the team is trying to do."
Cueto went through similar injury frustration in 2007 and ended up playing in the final having missed the previous two matches with a hamstring injury.
"I won't sit here and lie and say I wasn't disappointed (to miss out last week). I was hugely disappointed. Experience is telling me it is a long competition," said Cueto.
"Anything can happen really. In 2007 I played in the pool stages and got a hamstring injury against Tonga in the final game.
"I missed the quarter against Australia and the semi against France. Josh (Lewsey) pulled his hamstring in the semi and I got in for the final of the World Cup.
"It is such a crazy competition. You can't compare it to anything else.
"One minute you are at the bottom of the pile and suicidal, for what of a better phrase, and the next minute you are playing in a World Cup final and top of the pile."
Johnson confirmed the versatile Matt Banahan is also an option to replace to Armitage, although he may be required to cover the centres if Tindall fails to recover from a dead leg.
England's concerns over the fitness of Jonny Wilkinson continued for another day as they wait to discover whether the arm injury he suffered against Scotland will rule him out of the France game.
"It's a shame to lose Delon as he has played well but these things happen and we have some quality wings in Mark Cueto and Matt Banahan to chose from," Johnson said.
"Both (Tindall and Wilkinson) are responding to treatment so we will just have to see where they are up to when we get back into weights and light training tomorrow."
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