McNamara told BBC North West Sport: "We're a way off finalising the squad.
"We've got a fair idea of the players that are potentially going to be part of our group, but there are always one or two that come in from nowhere."
He continued: "Look at Chris Hill. He came from nowhere last year.
Chris Hill's incredible rise
In 2011, Chris Hill was playing for Championship outfit Leigh Centurions. By the end of 2012, he had won a Challenge Cup and played in a Grand Final for Warrington, as well as gaining his first England caps in the Autumn International Series.
"He was playing for Leigh in the Championship, then he started playing for Warrington, he got in the Dream Team, he got into the England team and
performed really well."
The second, against unconfirmed opponents on 19 October, takes place seven days before England play their opening pool fixture against Australia.
With the tournament commencing shortly after a punishing Super League season, McNamara is bracing himself for the loss of some key names before he announces his final party on 1 October.
The former Bradford Bulls coach added: "We're foolish if we think we'll get through to the end of the season and everyone will be available. That doesn't happen because the competition's too tough.
"It's that balance between tearing into each other, to create that pressure on each other in game situations, and we need them to be looked after by their clubs as well.
Highlights: England win Autumn International Series
"Hopefully, with a bit of luck, we'll come out with no injuries."
Speaking at the launch of the new domestic campaign in Manchester, McNamara said it was vital the
Super League is competitive this year,
in order to give him the best chance of leading England to World Cup glory.
"It's imperative," he said. "We've got a lot of things within our programme to give our players the best chance of performing in the World Cup.
"But without doubt, the best way of improving our chances is by the players really tearing into each other and creating a very good Super League competition. That will improve them out of sight.
"Hopefully they get handed over to us in October and they're ready to play."
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