Both sides have failed to win a game so far in their Pool, going down narrowly to Cardiff Blues and Edinburgh. For the losers on Saturday, their campaign will effectively be over.
Armitage will start at full-back with Casey making his return after recovering from an injury sustained pre-season - he comes in on the bench. Paul Hodgson will be aiming to rack up his 200th appearance for the Exiles if he takes to the pitch from the bench.
Clarke Dermody returns to the starting line-up as captain, with Darren Allinson at scrum-half. Bryn Evans will join Matt Garvey in the second row. Ofisa Treviranus also starts in the pack alongside Declan Danaher and Chris Hala'ufia. David Paice comes in as hooker.
"We have to win each of the four games left in the pool stages of this tournament - it is as simple as that. We have prepared well and know it will be a challenge but that is what we relish. The whole squad is up for the task," said London Irish head coach Toby Booth.
Meanwhile, injuries have forced Racing to name a somewhat makeshift side though Pumas playmaker Juan Martin Hernandez starts at fly-half and former France captain Lionel Nallet features in the second row. Sebastien Chabal start on the bench.
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