The South African leader said the unity government was essentially a transitional vehicle whose main task would be to salvage the economy that has crumbled under the world’s highest inflation rate of over 231-million percent.
"Essentially the inclusive government is a transitional authority," said Motlanthe who is also the current chair of the regional Southern African Development Community (SADC) that facilitated Zimbabwe's power-sharing deal.
"The main tasks were really to stabilise the political situation and embark on economic recovery for the country," Motlanthe told journalists on the sidelines of the African Union summit in the Ethiopian capital.
SADC leaders resolved after a marathon summit last week that President Robert Mugabe's ruling Zanu (PF) party and the opposition should form a government of national unity outlined under a September power-sharing agreement to end months of political strife following disputed elections in March.
But many are skeptical that the unity government will last or work, given the mistrust and deep-seated animosity between Mugabe and opposition Movement for Democratic Change (MDC) party leader Morgan Tsvangirai.
"Depending on how it goes, and whether by agreement this inclusive government decides to call early elections . . . that’s a matter that they would be able to resolve as Zimbabweans," Motlanthe said.
Once a regional breadbasket, Zimbabwe is in the grip of a severe economic crisis and food shortages that Mugabe blames on poor weather and Western sanctions he says have hampered importation of fertilizers, seed, and other farming inputs.
But critics blame Zimbabwe’s troubles on repression and wrong polices by Mugabe such as his land reforms that displaced established white commercial farmers and replaced them with either incompetent or inadequately funded black farmers, leading to a massive drop in farm production.
The economic collapse has also made it difficult for aid agencies to work in Zimbabwe because of high prices for supplies, troubles in paying salaries, difficulty in accessing food for staff and fuel shortages.
With its value eroded by the world's highest inflation, the Zimbabwe dollar is nearly worthless and every worker, consumer or trader is increasingly shunning the currency in favour of hard cash.
Crumbling sanitation systems have unleashed a deadly cholera outbreak that the UN says has killed 3 229 people and infected 62 909 others across the country – the worst death toll in Africa from an outbreak of the normally preventable disease in 15 years.
Zimbabweans hope a unity government will help ease the political situation and allow the country to focus on tackling the economic crisis and humanitarian crisis that is seen in acute food shortages, hyperinflation and deepening poverty and an outbreak of diseases such cholera and anthrax.
"Once the inclusive government is in place, part of the responsibility will be to go and try to attract investments, particularly in the infrastructure," Motlanthe said.
"The telling part in terms of the implementation of the economic recovery plan would be to get investment in infrastructure," he said.
However, there has been lukewarm response to the unity government from Western governments whose financial support is critical to any programme to revive Zimbabwe's comatose economy.
The United States and Britain, who are Zimbabwe's biggest donors, have said they will adopt a wait-and-see attitude to the unity government, with UK Africa minister Mark Malloch-Brown saying on Tuesday that London would maintain sanctions against Mugabe and his top lieutenants.
Malloch-Brown told the BBC Britain was sceptical about the new coalition government in Zimbabwe but believed it should be given assistance to tackle the humanitarian crisis in the country.
The UK government would continue to give such support but would not lift sanctions on Mugabe and his top officials until it was convinced there were committed to power-sharing and democratic change.