The tight-head replaces Harlequins prop Will Collier and will be back in white for the first time since 2010, having been part of a dominant front row unit at the Saints this season so far.
And Mercey will be among familiar company up front too, Alex Waller, Calum Clark and Sam Dickinson all retained as the Saxons welcome the Wolfhounds to Gloucester’s Kingsholm.
Kick off on Saturday afternoon is at 5pm and the fixture is due to be shown live on Sky Sports 1HD, immediately after the Saints’ clash with Dragons in the LV= Cup on the same channel.
UPDATED ENGLAND SAXONS SQUAD
Nathan Catt (Bath Rugby)
Calum Clark (Northampton Saints)
Sam Dickinson (Northampton Saints)
Jamie George (Saracens)
George Kruis (Saracens)
Charlie Matthews (Harlequins)
Tom Mercey (Northampton Saints)
Luke Wallace (Harlequins)
Alex Waller (Northampton Saints)
Scott Wilson (Newcastle Falcons)
Freddie Burns (Gloucester Rugby)
Elliot Daly (London Wasps)
Sam Hill (Exeter Chiefs)
Matt Hopper (Harlequins)
Dave Lewis (Exeter Chiefs)
Rob Miller (Sale Sharks)
Joe Simpson (London Wasps)
Henry Slade (Exeter Chiefs)
Semesa Rokoduguni (Bath Rugby)
Tom Varndell (London Wasps)