Wickremesinghe told The Sunday Morning in an exclusive brief interview that he remained confident that he would have the numbers to show in Parliament and remain as Prime Minister.
Mr Rajapaksa's supporters have accused Mr Wickremesinghe of selling Sri Lanka's assets, citing a 99-year lease agreement his government struck with Hong Kong conglomerate China Merchants Port Holdings Co Ltd last year to operate a failing port developed with Chinese debt during Mr Rajapaksa's decade-long presidency.
Thousands of Sri Lankans are marching in support of a new government under the country's former strongman.
Sirisena announced on Sunday that parliament would reconvene on November 14, a week later than he had promised, prolonging the power struggle that has crippled the country.
Al-Jazeera reported, while quoting sources, that the decision drew strong criticism from the supporters of the country's ousted Prime Minister Ranil Wickremesinghe 's United National Party (UNP), who even termed the order to be "too late".
"I wish to emphasize that I am compelled to accept the status that existed previously until such time that they (the pro-Wickremesinghe lawmakers) and the new political alliance prove their majority in Parliament", he said.
"The majority of the members are of the view that the changes done in the parliament are unconstitutional and against the traditions", the speaker of parliament, Karu Jayasuriya, said in a statement.
Rashida Tlaib becomes first Muslim woman elected to Congress
Researchers found that people identifying as Republicans were most likely to hold negative ideas about Islam and Muslims. Tlaib's win comes after the 2016 election of Trump spurred a record number of women to put their names on the ballot.
Addressing the "Jana Mahimaya" political rally organized by UPFA and several political groups in support of the present government at the Parliamentary Roundabout today, President Sirisena said the appointment was made in accordance with the Constitution and within the legal framework of the country.
Mr Sirisena and Mr Rajapaksa arrived at the rally amid loud cheers.
Sirisena was supported by Wickremesinghe three years ago but the two men have gradually drifted apart, leading to Wickremesinghe's shock sacking last month.
Rajapaksa, who was the president at the time, has repeatedly said that the war was against the LTTE and not against the Tamil community. Parliamentarians are also favouring the ousted PM along with the speaker.
The UNP has handed over a motion of no trust against Rajapaksa.
He said he hadn't received any money yet.
The unity government was formed in 2015 when Sirisena was elected President with Wickremesinghe's support, ending a almost decade-long rule by Rajapaksa.
He resigned as the deputy minister of Labour and Foreign Employment to leave the Sirisena camp and join Wickremesinghe.