There are still tickets available for tonight's game from Spar (Donnybrook) and the Leinster Rugby Store and cash will also be taken at the turnstiles.
Casey, who had a bicep operation and visited a knee specialist Stateside to get fully fit for the new season, will be an interested spectator as the Exiles take on his old club
Speaking to the Reading Chronicle, he said: "I'm still on my way back. I am back in full contact training this week.
"It's great to be back but the Leinster game has come too soon. I will still be travelling across to see some friends and family.
"It is disappointing to miss out, but being ready for the start of the Guinness Premiership is the most important thing.
"Right now it looks like the Saracens game (on September 5) could be a possibility, so that is what I'm aiming for."
The Maynooth man was included in the travelling party for the Leinster game, with London Irish coach Toby Booth selecting 14 international players in his 25-man squad.
Booth said: "With the start date for both the Guinness Premiership and Magners League just over a week away, both ourselves and Leinster are focused on getting our seasons up and running successfully.
"This is an important match as it provides the final opportunity to test combinations and plans in a real match situation.
"We're delighted to get the chance to play the current European champions at home, although I've no doubt Friday's match will be very different to the one that will take place when the two teams meet in Dublin in the Heineken Cup on October 9."
London Irish launched their new kit for the 2009/10 campaign at Elverys St. Stephen's Green store in Dublin yesterday.
You can purchase London Irish's new home and away jersey through the IRFU's own webshop, www.IrishRugbyShop.ie.
Click here for a direct link to the London Irish range.
LONDON IRISH Squad (v Leinster) - Backs (11): Delon Armitage*, Charlie Gower, Peter Hewat, Paul Hodgson*, Alfredo Lalanne*, Ryan Lamb, Jamie Lennard, Seilala Mapusua*, Peter Richards*, Sailosi Tagicakibau*, Marcus Watson.
Forwards (14): Steffon Armitage*, James Buckland, Danie Coetzee*, Declan Danaher, Clarke Dermody*, Chris Hala'Ufia*, Paulica Ion*, Nick Kennedy*, Dan Murphy, Andy Perry, Faan Rautenbach*, Kieran Roche, George Stowers*, Richard Thorpe.
* denotes international player