"South Africa, together with Turkey, are at the bottom of the ladder when it comes to policy credibility," Lutfey Siddiqi, visiting professor-in-practice at the London School of Economics (LSE), told CNBC's "Squawk Box Europe" on Wednesday.
"Credibility will be built based on action, not just on rhetoric," he added.
On Tuesday, the International Monetary Fund (IMF) cut its global economic growth forecasts for 2018 and 2019 by 0.2 percentage points to 3.7 percent. It also lowered projections for the increase in goods and services trade worldwide.
The Washington D.C.-based institute also warned the U.S.-China trade war was already showing signs of negatively impacting global growth, while emerging markets were struggling with tighter liquidity and capital outflows.
The IMF also said the balance of the risks were now tilted towards the downside, with rising interest rates likely to hurt emerging markets further at a time when U.S.-led demand growth would soon start to slow as some tax cuts expire.
To be sure, the normalizing of U.S. interest rates could pressure some emerging markets with capital outflows — most notably Turkey, South Africa, Argentina, Brazil, Mexico and Indonesia.
LSE's Siddiqi argued that even though a country like Indonesia was in more a precarious economic position than many other emerging markets, its central bank's response "feels like it's part of a plan" to improve the situation.
"Whereas in the case of South Africa — or Turkey certainly — it feels like (they are) kicking and screaming and dragging a change … Do they really want to do it or not?"
The last few months have been nothing short of a rollercoaster ride for Turkish assets, which have seen a massive sell-off thanks to investor concerns over the central bank's independence and a diplomatic fight between Turkish President Recep Erdogan and Washington.
Fears of contagion and a wider emerging markets shake-up culminated in Turkey's central bank increasing its benchmark interest rate to 24 percent last month. The move exceeded market expectations and helped to steady the lira against the dollar.
Nonetheless, the Turkish currency has fallen around 40 percent against the greenback this year.
Meanwhile, South Africa's finance minister Nhlanhla Nene resigned on Tuesday. It comes after Nene admitted meeting with members of the Gupta family — who have been accused of corruption.
Nene has since been replaced by former central bank governor Tito Mboweni. His appointment constitutes the fifth change of finance minister since 2014.
South Africa's rand and bond markets have reacted badly to the upheavals, though the currency firmed on the news of Nene's departure. The rand stood at 14.6288 against the U.S. dollar on Wednesday, down more than 15 percent year-to-date.
"I remember in 2015, (South Africa) had three finance ministers over a span over four days so while we welcome the appointment of a new finance minister, credibility is not built in a day," Siddiqi said.
Moody's rating agency is scheduled to review its stance on South Africa on Friday, with Mboweni's appointment seen restoring stability for an economy now in recession.
— CNBC's Natasha Turak contributed to this report.