Evgeni Tanchev's opinion came out on the same day when Poland ruled that the country's human rights ombudsman - a critic of the government - must leave office at the end of his term.
The Court of Justice of the European Union (CJEU) says that Poland has breached the independence of its own judicial system.
Europe's highest court said that two recently appointed Supreme Court judges are not impartial nor are they independent, and breach EU law.
The statement was made in an opinion by the CJEU Advocate General Evgeni Tanchev on Thursday.
"A court chamber does not constitute an independent and impartial tribunal, within the meaning of EU law, when ... the manner of appointment of its members are capable of giving rise to legitimate doubts," Tanchev said.
He added that Poland's President Andrzej Duda had committed a blatant violation of national law by appointing the two district judges - Monika Frackowiak and Waldemar Zurek. Both are partially contesting their transfer to Poland's Supreme Court.
The CJEU is due to rule on the case but doesn’t have to follow Tanchev’s reasoning.
Brussels has been locked in a legal battle with Warsaw over the so-called "rule of law" and the appointment of judges.
Poland's ruling conservative Law and Justice Party (PiS) has been restructuring the country's judiciary for years, despite international criticism, putting judges under pressure.
"The attack on the national judiciary threatens the operation of the European Union," said Kriszta Kovacs, a Marie Curie Fellow at WZB Berlin.
Leading government critic could be forced to leave human rights role
Meanwhile, Poland's top court has ruled that it is unconstitutional for the country's independent human rights ombudsman to remain in the job indefinitely after his term has expired.
The ruling by the pro-government Constitutional Tribunal paves the way for the removal of the current acting ombudsman, Adam Bodnar.
The Tribunal said that the Polish law allowing the ombudsman to remain in office was poorly worded and lacked time limits.
But Council of Europe Human Rights Commissioner Dunja Mijatovic said the decision "creates a worrying gap in the functioning of the institution of Ombudsman between mandates and in the protection of human rights in Poland."
Bodnar has been a figure of frustration for PiS, frequently criticising the government over women’s and minority rights.
Speaking to senators on Wednesday ahead of the decision, Bodnar said that "the current political power is seeking to extend its influence also in this area of activity of public institutions".