2005 Lion Moody broke his ankle in training with his club Leicester and could be sidelined for as many as three months.
"Steffon has been in very good form for London Irish this season and deserves his opportunity," said England team manager and former Lions skipper Martin Johnson.
Wasps flanker Joe Worsley, Bath wing Matt Banahan and Leicester and Lions lock Ben Kay have also been named as cover for Tom Rees, Ugo Monye and Tom Palmer respectively.
Palmer is set to miss the entire Six Nations campaign, while Rees is out of at least the first three matches and Monye has not travelled with the squad to their training camp in Portugal as he instead receives treatment in England on a back problem.
Despite being knocked out during Bath's Heineken Cup encounter with Toulouse on Sunday, openside flanker Michael Lipman has also been added to the squad as cover.
Armitage's inclusion in the senior squad sees Harlequins captain Will Skinner join up with the Saxons.
Forwards: Tim Payne (Wasps), Andrew Sheridan (Sale), Phil Vickery (Wasps), Julian White (Leicester), George Chuter (Leicester), Dylan Hartley (Northampton), Lee Mears (Bath), Steve Borthwick (Saracens), Nick Kennedy (London Irish), Tom Palmer (Wasps), Simon Shaw (Wasps), Steffon Armitage (London Irish), Tom Croft (Leicester), James Haskell (Wasps), Tom Rees (Wasps), Nick Easter (Harlequins), Luke Narraway (Gloucester)
Backs: Delon Armitage (London Irish), Olly Morgan (Gloucester), Mathew Tait (Sale), Mark Cueto (Sale), Paul Sackey (Wasps), Ugo Monye (Harlequins), Jamie Noon (Newcastle), Mike Tindall (Gloucester), Riki Flutey (Wasps), Shane Geraghty (London Irish), Danny Cipriani (Wasps), Toby Flood (Leicester), Danny Care (Harlequins), Harry Ellis (Leicester), Ben Foden (Northampton)
Also training with the senior squad: Matt Banahan (Bath), Ben Kay (Leicester), Michael Lipman (Bath), Joe Worsley (Wasps)