New premier Hichem Mechichi, 46, an independent, now has a month to form a government capable of winning a confidence vote in parliament by a simple majority, or the president will dissolve parliament and call for another election with urgent economic decisions hanging over Tunisia.
The resignation of Fakhfakh this month rippled through parliament, where parties are seeking a no confidence motion against Speaker Rached Ghannouchi, the leader of the moderate Islamist Ennahda party. A session is scheduled for Thursday but with Saied choosing Mechichi, the result of any vote in parliament not guaranteed.
Mechichi is seen close to Saied and served as the president's adviser. He was also member of the National Commission of Investigation on Corruption founded in 2011.
But political analysts says Mechichi has no economic background at a time when international lenders are asking Tunisia to make painful reforms.
Western countries have hailed Tunisia for its comparatively successful transition to democracy since the 2011 revolution that ended decades of autocratic rule despite periodic crises.
Many Tunisians are frustrated with economic stagnation, a decline in living standards and decay in public services while political parties often seem more focused on staying in office instead of tackling problems.
The novel coronavirus pandemic has made things worse. Tunisia now expects the economy to shrink by 6.5% this year and forecasts a deficit equivalent to 7% of gross domestic product.
It has asked four countries to delay debt repayments.The last parliamentary election in October led to a chamber in which no party held more than a quarter of the seats, complicating efforts to form a stable government.