Mumps is a contagious viral infection, and the All Blacks have been wary of other squad members catching it. Recovery time usually ranges from anywhere between two to three weeks.
This is the second mumps case to hit this squad. Rookie centre Jack Goodhue missed the opening match against the Baabaas with the same illness, staying at home to recover, before rejoining the team in London last Friday.
"It's unrelated to Jack Goodhue because they haven't been in contact," All Blacks coach Steve Hansen said as the team ventured from London to Paris on the train. "It's obviously come out of Auckland somewhere. It will be touch and go. He trained this morning it will depend on how well he comes through the week we'll wait and see."
Hansen downplayed fears the virus may infect others.
"Doc has done a great job he isolated him so no we're not worried."
Losing Ioane would be a blow. The 20-year-old has been a star on the left wing for the All Blacks, scoring nine tries in 10 appearances since moving past Julian Savea.
Ioane's form has been so compelling that he is considered a strong favourite for World Rugby breakthrough player of the year.
Seta Tamanivalu, who made his first start in the No 11 jersey against the Baabaas and performed well there, would be favoured to retain that role if Ioane is ruled out.