Lomu was left out of the Wellington side to play Southland after missing two training sessions because of tightness in his calf muscles.
His manager, Phil Kingsley-Jones, says the problem may be related to the dialysis treatment Lomu is having for kidney disease.
The big wing trained on Tuesday morning, but sat out the afternoon run.
The Wellington team's medical staff told him it was not worth him travelling from Auckland for yesterday's training where the team was announced.
Lomu never appeared to have a realistic chance of making the 30-man World Cup squad.
Apart from whether his form was good enough or his game suited to the test plan, he has had little time to get his fitness right.
He has one more NPC round, in which Wellington will play Waikato, to show his capabilities before the World Cup squad is announced at the Ponsonby rugby club on August 25.
Kingsley-Jones said Lomu's comeback had not been helped by his having to return to Auckland for his daily dialysis treatment.
His Wellington home is being "plumbed out" for the installation of a machine at the end of this month.
"I've never said he should be an All Black again, but this is Jonah's dream," said Kingsley-Jones.
"Who knows? No one has ever tried this before. It's like being the first man to land on the moon. I believe the All Blacks need him."
Kingsley-Jones said All Blacks doctor John Mayhew had ascertained that treatment would be available in Melbourne, the All Blacks' base during the World Cup in October and November.
Mehrtens has succumbed to a calf strain incurred in training this week.
All Blacks coach John Mitchell had told the record test points scorer about changes needed in his game - more aggressive defence and taking the ball to defensive lines.
It always seemed a tall order for Mehrtens to alter his game at this stage of his career.
Like Lomu, he has one NPC game left to show the selectors his wares.