Stephen Jones, who stepped down as Santander UK's chief financial officer several months ago, and at least one internal candidate are thought to have been shortlisted.
Booker, who joined Co-op in 2013 following an accounting scandal that almost caused the bank's collapse, has a contract that expires at the end of 2016 and is widely expected not to extend it.
The bank is still struggling to deal with the £1.5bn blackhole that was uncovered in its finances that prevented it from buying 630 branches from Lloyds Banking Group that had to be spun out as TSB.
Co-op Bank will give investors an update over Booker's replacement after the release of its results for 2015 tomorrow morning, City A.M. understands.
In the first six months of 2015 the bank recorded a loss of more than £200m, and is thought unlikely to return to health for some years.
It is now just 20 per cent owned by the Co-operative Group after it was bailed out by two US hedgefunds following the scandal.
Booker, who previously was an executive at HSBC, took over from former Co-op Bank boss Barry Tootle after the balance sheet problems were reveal. Tootle was fined and banned from the financial industry in January this year.
The banks woes worsened when its then chairman's, Reverend Paul Flowers (dubbed the crystal methodist), financial competence was brought into question and his history of drug-taking and use of prostitutes was exposed following a newspaper investigation.
The news of Niall Booker's replacements was first reported by Sky News.
Co-op Bank declined to comment.