BJP vice president Mukhtar Abbas Naqvi said that Sinha's remarks were "very unfortunate".
"The matter is sub judice and the investigative agency is saying that there may not be concrete evidence," Naqvi told IANS.
"The matter should be properly investigated," he said.
Sinha had told a newspaper Wednesday that it would be difficult to prove that Bansal was involved in the cash-for-jobs controversy as the former minister has a strong defence.
Bansal was forced to quit as railway minister earlier this month after a bribery controversy surrounding his nephew Vijay Singla.
The CBI arrested Singla May 3 and recovered Rs 90 lakh from him, which was allegedly paid to him by a senior railway officer to get a lucrative posting.