WASHINGTON—The International Monetary Fund had a sobering message for Greece this weekend: Even if the country secures debt relief from its European creditors—a question that is by no means assured with bailout talks still deadlocked—the nation still needs even more painful economic overhauls than currently planned.

Seven years into an economic crisis and another near-term financial emergency looming, that is a message no Greek wants to hear and a key reason why the IMF is also urging Germany and Athens’ other European creditors...