The Goa Crime branch is probing the death of Dias, whose body was found in a river at his village St Estevam near Panaji on November 7.
Dias, who was part of almost all environment protection movements in the coastal state, was last seen with the minor.
The Juvenile Court yesterday rejected the application of the Crime Branch to subject the minor witness to a lie detection test even as his parents had given their consent for the same.
Meanwhile, the Judicial Magistrate First Class (JMFC), Panjim has already allowed a lie-detector test on another prime witness in the case.
Superintendent of Crime Branch Karthik Kashyap said they are ready to conduct the test even tomorrow on the first (major) witness but the date will be set based on the availability of the lab.
The death of Fr Dias was initially investigated by local police but owing to pressure by various sections the probe was handed over to crime branch.