Igga, Speaker of South Sudan's Legislative Assembly since 2005, appears a choice to balance ethnic diversity among leaders of the young nation and to shore up support ahead of elections due in 2015.
Deputy chairman of the ruling Sudan People's Liberation Movement (SPLM), Igga comes from the Bari tribe, whose homeland surrounds the capital Juba.
Last month Kiir took the surprise move of sacking the entire cabinet as well as Machar, a charismatic but controversial leader who fought on both sides of Sudan's brutal 1983-2005 civil war.
However, Igga's appointment is seen as a way for Kiir to stem critics who argue too much power has been concentrated in the Dinka people, and to reach out to citizens in the Equatoria region in the southern half of the country where Igga is from.
"In the case of the tensions within the SPLM, there are immediate concerns about a repeat of the historical split between the Dinka and the Nuer," the United States Institute of Peace said in a recent report.
It is not clear who will succeed Igga in the important post of speaker.
These days smartly dressed in well cut suits and colourful ties, Igga is a veteran of the rebel force that fought Khartoum in the more than two decades long civil war.
Igga, 64, a Roman Catholic, grew up during Sudan's first north-south civil war, an on-off conflict that stretched from independence from Britain in 1956 to a 1972 peace deal.
Returning to Sudan he joined the rebels in 1985, two years after the rebellion began. He received military training in Ethiopia and Cuba and rose swiftly as a commander, retiring active service as a general in 2005 to enter politics.
