The restoration of Mir Jafar to the Subahdari of Bengal in 1763 opened the prospect of a new career for Reza Khan. Mir Jafar was proclaimed Subahdar at Calcutta on 7 July and a declaration of war was made against Mir Qasim for alleged open acts of hostility to the English. The declaration invited ‘all manner of persons’ to the standard of Mir Jafar and to assist him in defeating the designs of Mir Qasim. By 15 August Reza Khan had reached Dacca to take up his new appointment as Naib Nazim or Deputy governor.

Though the Dacca appointment was a very high distinction Reza Khan was not initially very enthusiastic about it. Two main reasons made it unwelcome to him: the fear of the machinations of Nandkumar against him, and the disorders and confusion at Dacca.

Nandkumar had become the principal minister of Mir Jafar immediately on the latter's restoration. Despite a very strong personal dislike of Hastings and Vansittart for Nandkumar the English had to agree to the Nawab's demand for his appointment. Mir Jafar was insistent, possibly, because Nandkumar, besides being a fellow sufferer since Vansittart's arrival in 1760, and his secret agent working against Mir Qasim's nizamat, was, to him, an expert on the English in Bengal. With a war in hand against Mir Qasim the English had to oblige Mir Jafar, particularly because the number of alternatives had also been reduced by Mir Qasim. Mir Qasim, ‘the best Mutseddee in his own court’ had taken away with him to Bihar ‘every Mutseddee and every officer in whom he could repose the least confidence’ and he trusted none.