Rakhmatullo Alibaev suffered a broken nose, a concussion, and trauma to the brain as a result of the attack, which took place on 24 January.
Alibaev, who was visiting the opposition party Ozod Dehqonlar (Free Peasants) at the time, had opened the door to a man who claimed to be the brother of the party's leader Nigora Hidoyatova.
The attack came on the eve of the opening of a court case against Hidoyatova.
HRW believes the beating appeared timed to send a message to those who might monitor her trial.
Alibaev has monitored trials of independent Muslims charged with religious extremism for a number of years.
HRW noted that the attack appears to be the latest in a growing number of attacks on human rights defenders.