Grandi's appeal also comes after his visit to the Asian nation that is struggling with growing violence, a US pullout and flagging peace talks.
Grandi said the future of millions of Afghans depends on the outcome of peace negotiations and on the international community's commitment to develop the country, including at the two-day donors conference in Geneva.
"Failure on either account would see Afghanistan slide backwards with disastrous consequences, including further displacement possibly on a large scale," Grandi warned in a statement.
He said the nearly 300,000 Afghans who have been displaced inside the country because of conflict this year remain in "acute need" of humanitarian support.
The same goes for the nearly three million previously displaced and the nine million people who have lost their livelihoods due to the Covid-19 crisis.
Grandi also said it was urgent to conclude peace negotiations between the Afghan government and the Taliban, which are fighting to topple it and retake the power they lost during the US-led invasion of Afghanistan in 2001.
The peace talks are flagging and a withdrawal of some of the US forces that the Trump administration announced this week are further weakening the Kabul government.
Grandi’s comments came as participants at the virtual conference in Geneva told Reuters that donors will introduce tight political and human right conditions on the reduced amount they pledge Kabul, whose fragile economy relies heavily on foreign aid.
The moves are aimed at protecting peace talks between Afghan government and Taliban rebels, which remain stalled, as well pushing the government to ensure more effective aid distribution.
The total amount could be cut by between 15% to 20% on the last Afghanistan conference held in 2016, a senior Western diplomat told Reuters. At the time, donors agreed to give the Afghan government $15.2 billion for the period between 2017 and 2020, equivalent to $3.8 billion a year.
"This is the best countries can offer amid the domestic challenge of managing a pandemic," the diplomat said.
According to a report by the World Bank, coronavirus-related impacts will cause the Afghan economy to contract by at least 5.5% year.
Hosted by Geneva, the 2020 virtual event will bring together ministers from 70 governments and officials from humanitarian organizations, who will pledge funds to protect development projects.
Afghan President Ashraf Ghani is expected to present a peace and development plan meant to allocate funds to the projects, safeguard millions of jobs and protect democratic institutions.
This meeting comes as US President Donald Trump announced last week that his country's troop presence would be reduced to 2,500 from 4,500 by mid-January.
Meanwhile, violence continues unabated across much of Afghanistan with a series of rocket attacks Saturday in Kabul, which killed at least eight people.
The government continues to blame the Taliban for these attacks, while the group denies involvement.
Agencies contributed to this report.