Ashok Mehta, a political analyst who once led the Indian peacekeeping forces in Sri Lanka, said that Fonseka had proved himself as one of Sri Lanka's most capable army commanders.
A soldier since 1970, Sarath Fonseka led the military to victory over the Tamil Tigers in May this year
He resigned from his defence chief post last week, accusing the president of "sidelining him" despite his contribution to the war effort
His military strategy, though victorious, prompted allegations of human rights abuses and high civilian casualties
On a visit to the US this month, there was speculation that US officials would question him over alleged war crimes, but that did not happen
Fonseka was wounded several times on the frontline and nearly killed in 2006 by a Tamil Tiger suicide bomber
"This is the first time that an army which had the reputation of being constantly badgered and bullied by the LTTE has actually managed to comprehensively defeat - in fact rout - the Liberation Tigers of Tamil Eelam," he told Al Jazeera.
"I think if you really count the architects of this military victory on your fingers, then there are three - the first is of course Mahinda Rajapaksa, the second is the defence secretary, and the third is Fonseka.
I think Fonseka must take the honours for the military victory on the ground, but, as we know, a military victory can only be orchestrated by the political will and the people's support in the country."
Sri Lanka's president on Monday promoted his air force commander to replace Fonseka.
Air Chief Marshal Roshan Goonetileke was made the new chief of defence staff in addition to his duties as head of the air force, according to an official from Rajapaksa's office.
On Sunday, Rajapaksa postponed an expected announcement of the schedules for early presidential and parliamentary elections due in April without giving a reason.
Political analysts say Fonseka, who spent 39 years in the army, could split Rajapaksa's voter base by attracting those happy with the victory in the war.
That could be unwelcome news for potential investors as some opposition parties in the country are considered less "pro-business" than Rajapaksa's coalition.
Fonseka was granted a near free rein with nearly unbridled power as he headed the war effort with his former colleague, Gotabaya Rajapaksa, the defence secretary and the president's brother.
They defeated the Tami Tigers in a 34-month campaign criticised by human rights groups as unnecessarily brutal and costly in civilian casualties.