Nonu was not invited to be part of the 41-man group which assembled in Wellington on Monday, but was offered a ray of hope when head coach Steve Hansen said the veteran midfielder would "be considered like anybody else".
Hansen said the selectors had decided to invite only players involved in the All Blacks' 2018 campaign.
"There are some guys who aren't here that, if you were picking it on form, probably would be here, but the rules are the rules and we are all playing by them."
Nonu - now 36 - played the last of his 103 tests for the All Blacks in the 2015 Rugby World Cup final before spending three seasons with French club Toulon.
He returned to New Zealand late last year and signed with the Blues. He has been in vintage touch in Super Rugby outings this season.
Nonu's form has not been lost on Hansen.
"He's just been Ma'a, hasn't he? You get the subtle touches and you get the brutality, as LB [Chiefs All Black midfielder Anton Lienert-Brown] found out in the weekend."
Hansen said no one knew "what [Nonu] was going to come back like".
"I just like the fact that he has been brave enough to come back, and he will be considered like anybody else.
"We know he can play terrific rugby and if he can do that on a weekly basis at Super level then we have to make decisions.
"There are lot of good players in that position, so some good people are going to miss out."
Nonu faces competition from established All Blacks midfielders Ryan Crotty, Sonny Bill Williams, Jack Goodhue and Anton Lienert-Brown, as well as Hurricanes' second five-eighths Ngani Laumape.
Of that group, only Williams has World Cup experience, after playing alongside Nonu in the 2011 and 2015 gold medal campaigns.
Nonu has played at three World Cups, with his first the tournament in Australia in 2003 - the year of his All Blacks' debut as a 21-year-old.
He was dropped for the 2007 Rugby World Cup, but returned to become an All Blacks regular from 2008 until his departure for France.
The Wellington-born former Hurricanes stalwart was one of the All Blacks' best performers in the 2015 title-winning campaign.