The battle for the inside centre jersey is the most intense of any as Hansen weighs up the merits of some highly-accomplished performers against Blues stalwarts Nonu and Williams, who were involved in New Zealand’s triumphant campaigns of 2011 and 2015.
Nonu, 36, also took part in the 2003 World Cup, in the formative stages of a 103-Test career.
It was thought the 2015 final at Twickenham would be his swansong but, after three seasons with French club Toulon, he has produced some compelling displays in the last two rounds of Super Rugby.
Hansen acknowledged Nonu’s form before explaining why he wasn’t included in a 41-strong player “foundation day” in Wellington on Monday.
He said it had been previously decided that only players contracted in 2018 would take part.
“There’s some guys who aren’t here that if you were picking it on form, probably would be here,” he said.
Nonu had thrust himself into the mix.
“None of us knew what Ma’a was going to come back like,” Hansen said.
“He’s just being Ma’a isn’t he? You’re getting the subtle touches and you’re getting the brutality.
“We know he can play Test rugby. He’s just got to show us he’s got the ability to do that on a weekly basis at Super level. Then we have to make decisions.”
Cross-code star Williams had undergone knee surgery last week and will be sidelined for another month, adding to a litany of ailments over the last two years.
Hansen said the 33-year-old boasted attributes and experience that make him invaluable.
“If he gets back and plays four or five games, bingo, he’s going to play some good footy, isn’t he?
“It’s just a matter of staying on the park. Once he’s shown he can do that, he’s going to be a hard man to leave out.”
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