"The fracture had almost healed but my thumb remained swollen. So we wanted to have another X-ray, which revealed that there was a fracture," ESPNcricinfo quoted Tamim as saying.
"Probably there were two, but it didn't come out in the first scan. The finger has gone back into the split. I can't move it. The whole process has to start again," he added.
Meanwhile, the Pakistan T20 squad arrived in Dhaka on Saturday for the T20I series starting from 19 November.
Pakistan T20 Squad: Babar Azam (captain), Shadab Khan, Asif Ali, Fakhar Zaman, Haider Ali, Haris Rauf, Hasan Ali, Iftikhar Ahmed, Imad Wasim, Khushdil Shah, Mohammad Nawaz, Moha mmad Rizwan, Mohammad Wasim Jnr, Sarfaraz Ahmed, Shaheen Shah Afridi, Shahnawaz Dahani, Shoaib Malik, Usman Qadir.
Pakistan squad for the two Tests, which will be played from 26-30 November and 4-8 December in Chittagong and Dhaka, respectively will be announced in due course.