Leicester flanker Tom Croft and uncapped Northampton lock Christian Day have been added to England's senior training squad for the Six Nations.
The pair join the squad after Geoff Parling and Tom Wood were injured while playing for their clubs on Saturday.
England have a number of injury concerns, with Owen Farrell (knee), Kyle Eastmond (shoulder) and Brad Barritt (knee) also hurt.
Luther Burrell is expected to take part in training on Friday.
Burrell is recovering from the concussion he suffered in Northampton's loss against Racing Metro.
His team-mate, flanker Wood, injured his ankle during the defeat, while Leicester lock Parling left the field with a knee injury three minutes into their defeat by Ulster which Tigers director of rugby Richard Cockerill believes could keep him out for "three to four weeks".
A 34-man England training squad met on Monday to prepare for their opening Six Nations match against Wales in Cardiff a week on Friday, 6 February.
England head coach Stuart Lancaster said he would be able to provide an update on Farrell - who will see a specialist on Monday evening - and his other injured players on Tuesday.
"The players will be fully assessed once they get into camp and it's too early to give any definitive news on their status," he said.
The second row is likely to be an area of particular concern for Lancaster, who is already without locks Joe Launchbury (neck) and Ed Slater (knee) for the entire tournament, while Courtney Lawes has had minor ankle surgery which has ruled him out until the middle of the campaign at the earliest.
Bath's Dave Attwood, Leicester's Graham Kitchener and Saracens' George Kruis are competing for the two starting berths in Cardiff, along with Day.
The injury-plagued Croft, who has won 38 caps plus five for the Lions, last played for England in their record 30-3 defeat by Wales in Cardiff on the final day of the 2013 Six Nations, while Day, 31, made two appearances for the second-string Saxons in 2011.
Their promotions mean Bath back-rower Carl Fearns and Wasps lock James Gaskell come into the Saxons squad for Friday's match against Ireland Wolfhounds.
England senior training squad:
Props: Dan Cole (Leicester Tigers), Alex Corbisiero (Northampton Saints), Joe Marler (Harlequins), Mako Vunipola (Saracens), David Wilson (Bath Rugby); Hookers: Dylan Hartley (Northampton Saints), Tom Youngs (Leicester Tigers); Locks: Dave Attwood (Bath Rugby), Christian Day (Northampton Saints), Graham Kitchener (Leicester Tigers), George Kruis (Saracens), Geoff Parling (Leicester Tigers); Back-rows: Calum Clark (Northampton Saints), Tom Croft (Leicester Tigers), James Haskell (Wasps), Nick Easter (Harlequins), (Wasps), Chris Robshaw (Harlequins, captain), Billy Vunipola (Saracens), Tom Wood (Northampton Saints),
Back three: Mike Brown (Harlequins), Alex Goode (Saracens), Jonny May (Gloucester Rugby), Jack Nowell (Exeter Chiefs), Anthony Watson (Bath Rugby); Centres: Brad Barritt (Saracens), Luther Burrell (Northampton Saints), Kyle Eastmond (Bath Rugby), Jonathan Joseph (Bath Rugby), Billy Twelvetrees (Gloucester Rugby); Fly-halves: Danny Cipriani (Sale Sharks), Owen Farrell (Saracens), George Ford (Bath Rugby), Stephen Myler (Northampton Saints); Scrum-halves: Danny Care (Harlequins), Richard Wigglesworth (Saracens), Ben Youngs (Leicester Tigers).