"It's disappointing for him and for us but again it creates an opportunity for someone like Ryan Crotty once he is all cleared to play, and Malakai [Fekitoa] for that fact," Hansen said.
Crotty (fractured cheekbone) and Kaino (elbow) were injured during the All Blacks' 51-20 victory over Australia at Eden Park late last month.
Whitelock (ribs) and Messam (quad) suffered their ailments during last weekend's triumph over Argentina and were originally expected to be out for three to four weeks.
"It's good work by the medical crew to get them on the plane," Hansen said.
All four players will board the plane to Argentina next week as the All Blacks prepare for the final two games of the Rugby Championship.
They will tackle the Pumas in La Plata on Sunday, September 28 (NZT) before meeting the Springboks in Johannesburg the following week.
It's unclear at this stage whether the returning players will be available for both games or whether they need the extra week to recover before playing South Africa.
Hansen's squad for the two-game swing will be 30-strong and will include three hookers with Dane Coles expected to miss the game against South Africa as he awaits the birth of his child.
Chiefs rake Nathan Harris, who is yet to make his international debut, will go on the trip as the third hooker.
There's also the possibility another hooker may trade places with Coles on his return to provide further cover.
"We'll let you know all about that during the week," Hansen said.
Injured first-five Dan Carter (broken leg) will soon make his return to rugby via the ITM Cup with an eye to be fit for the 'dead' Bledisloe Cup encounter with Australia in Brisbane on October 18.
"He'll have a graded programme," Hansen said of Carter's workload at provincial level.
