"You can't take a country that's over-borrowed and make it more creditworthy by lending it more money," he said. "They're throwing Greece further and further and further in the hole by not addressing the problem directly and properly."

Asked when a Greek default could happen, Weinberg answered: "at High-Frequency, we are advising people to take their cell phones on their August vacation." He said a Greek default would be "harsh" for the euro .

Greek officials did not respond to CNBC.com requests for comment.

On Thursday, Nassim Taleb, professor and author of the bestselling book "The Black Swan," told CNBC that the economic situation today is drastically worse than a couple of years ago, and that the euro is doomed as a concept.

But famous investor Jim Rogers said now may be a time to buy the single European currency, as there are so many investors who are bearish about it that a rally may be in the making.

IMF and EU funds worth about 7.5 billion euros ($9 billion), crucial for Greece to be able to pay foreign debt, are due to be disbursed at the end of August, Weinberg said.

"Unless (Greeks) meet the quantified adjustment targets that they agreed to in the memorandum of understanding with the IMF, they won't get this money," he said, adding that his bet is that Greece will not meet the criteria.

EU Won't Let Default Happen

Under the memorandum of understanding, performance criteria include ceilings on the budget deficit, cutting government and social security spending, as well as revamping key public companies.

However, other analysts say the implications of a Greek default on the euro zone's financial institutions and economy are so great that the two institutions will disburse the funds even if the country does not fully meet the criteria.