"Sean is so important to the group and we wanted to give him every opportunity to be fit," added McNamara. "I'm very confident he will be fit."
England, who finished third in the last World Cup in 2008,
start their campaign on 26 October
against Australia in Cardiff before moving to Huddersfield, where they face Ireland on 2 November.
World Cup 2013
with a double-header at Cardiff's Millennium Stadium featuring England v Australia and Wales v Italy
Final at Old Trafford on
14 teams taking part: England, Australia, Fiji, Ireland, New Zealand, France, Samoa, Papua New Guinea, Scotland, Tonga, Italy, Wales, Cook Islands, United States
There are 28 games in total, taking place in England, Wales, Ireland and France
They round off their Group A campaign against Fiji in Hull on 9 November.
McNamara said: "We are fully aware that the tournament will be fiercely contested and that this squad will need to work extremely hard and play to a very high level to win it.
"As we have seen in previous tournaments, we will need all members of the squad ready to step up and make vital contributions.
"We feel that all 24 players have the ability to do that.
"To have a World Cup on home soil is a massive opportunity for the sport and I hope the public will turn up in their numbers and show their support for England."
New Zealand are the reigning champions, having beaten Australia 34-20 in the final five years ago.
Carl Ablett (Leeds), Tom Briscoe (Hull), George Burgess, Sam Burgess, Tom Burgess (all South Sydney), Rob Burrow (Leeds), Josh Charnley (Wigan), Rangi Chase (Castleford), Leroy Cudjoe (Huddersfield), Liam Farrell (Wigan), James Graham (Canterbury Bulldogs), Ryan Hall (Leeds), Zak Hardaker (Leeds), Chris Hill (Warrington), Gareth Hock (Widnes), Michael McIlorum (Wigan), Lee Mossop (Wigan), Sean O'Loughlin (Wigan), James Roby (St Helens), Kevin Sinfield (Leeds, capt), Sam Tomkins (Wigan), Kallum Watkins (Leeds), Ben Westwood (Warrington), Gareth Widdop (Melbourne).
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