Investors in Greek and other euro-denominated sovereign debt must accept that they are going to lose money unlike in past crises, says Mohamed El-Erian, CEO of Pimco, the world's largest bond fund

"This time around there's going to be a lot more burden sharing," El-Erian tells CNBC.

"I don't think the market has quite understood that it's not simply a matter of recapitalization, but decisions are going to have to be made about burden sharing, just like they're being made today in Greece. This is what's going to be different than '08 and '09."

Mohamed El-Erian (Associated Press photo)

Greece has agreed to tough austerity measures in return for aid from other European nations, although everyone must admit now that default — often branded more softly as restructuring — is becoming more likely.

"From day one, people knew that this program would not deliver outcomes," El-Erian says of the Greek austerity plan.

"This was not about Greece. This was about keeping Greece somewhat stable in order to strengthen the firewall, to strengthen the other countries."

Other experts agree that a Greek default is a question of "when" and not "if."

"A default is likely," says Wolfango Piccoli, director of the London office of the Eurasia Group, CNNMoney reports.

"At this stage, the question is about the timing."

Greek debt levels are sustainable, says Farid Abolfathi, senior director of the Risk Center at IHS Global, CNNMoney adds.

"No matter how much austerity, they will not be able to pay their creditors. At some point in time, they are going to default," Abolfathi says.