Earlier this month, a provisional deal was agreed between the country, the International Monetary Fund (IMF) and other eurozone member states to provide Greece with â¬45 billion.
But Erik Nielsen, chief European economist for Goldman Sachs, has warned this figure is nowhere near high enough to tackle Greece's economic woes, reports the Guardian.
"On my numbers, a one-year fully funded program needs to provide a minimum â¬50-55 billion, an 18-month program will require some â¬75 billion, and a three-year program a minimum â¬150 billion," he said in a note to investors.
"I think the latter number is out of reach even for the present political environment of generosity, so the debate is between â¬55 billion and â¬75 billion."
Mr Nielsen added that he believes that representatives from the IMF and the eurozone will already be debating how they will share the burden of delivering more financial aid.
He said he does not think it likely the eurozone countries will be able to come up with much more cash than the â¬30 billion they have already pledged to hand over to Greece.
Mr Nielsen went on to speculate that the IMF may be able to put together an additional funding program based on either providing â¬55 billion over the course of a year or â¬80 billion across an 18-month period.
Earlier this week, German finance minister Wolfgang Schauble warned that it is not yet certain the eurozone will provide the initial â¬30 billion of funding Greece is expecting.
He said that the provision of aid is dependent on Greece coming up with a detailed plan on how it intends to cut its budget deficit in the next few years.
By Claire Archer