"'He's done an ACL and looks to be out for eight to nine months which is a real shame for him", Hansen said.
Hansen described Nonu's return to the Blues and goal of making another World Cup four years after his last test as "brave" and said the selectors would have some hard decisions to make around the make-up of the midfield if the veteran's good form continued.
"There is no point us telling you because then you'll go and tell the world and they've got a head start. No".
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.
McKenzie was expected be one of the three All Blacks flyhalves selected for the World Cup, along with Beauden Barrett and Richie Mo'unga.
He returned to New Zealand late previous year and signed with the Blues. He has been in vintage touch in Super Rugby outings this season.
Hansen acknowledged Nonu's form before explaining why he wasn't included in a 41-strong player "foundation day" in Wellington on Monday.
"He's just been Ma'a, hasn't he?"
Nonu burrows over for a try in the Blues' loss to the Chiefs.
"None of us knew what Ma'a was going to come back like", Hansen said. I just like the fact that he's been courageous enough to come back and he'll be considered like everybody else. He's just got to show us he's got the ability to do that on a weekly basis at Super level. "As someone pointed out, there are a lot of good players in that position so there will be good people who miss out".
Nonu, 36, also took part in the 2003 World Cup, in the formative stages of a 103-Test career.