Closing New York City’s troubled jail complex on Rikers Island will take at least a decade and will require a big decline in the inmate population, a continued drop in the city’s already low crime rates, a wellspring of funding and political capital, according to a strikingly blunt proposal that Mayor Bill de Blasio intends to unveil on Thursday.

The plan, laid out in a 51-page report, solidifies the mayor’s stance on closing Rikers, a goal that he was reluctant to embrace and that a few years ago seemed politically and practically unfeasible. Mr. de Blasio came out publicly in support of closing Rikers only in April, after increasingly well-attended protests and calls from civic and political leaders, including the City Council speaker, who created an independent commission that unveiled its own plan for closing Rikers three months ago.

The report describes a “credible path” that “will not be easy,” a message intended for the mayor’s critics who have demanded that he move more quickly to shut down Rikers and end the abuses that continue to plague it.

“It would be much simpler for us to tell people what they want to hear and say we can achieve this goal quickly and easily, but we won’t do that,” Mr. de Blasio writes in the introduction. “Instead, we are realistic.”