Pimentel, guest at the weekly Kapihan sa Sulo forum, said allegations that Marcelo has a conflict of interest should take a back seat and allow him to have his day in his office.
"It is President Arroyos call and the food or the pudding is still on the table, so to speak," he said of the Presidents decision to pick Marcelo from the list of three nominees submitted to Malacañang by the Judicial and Bar Council from out of 22 applicants for Ombudsman.
The two others were chief state prosecutor Jovencito Zuño and Social Security Commission chairman Bernardino Abes.
The President picked Marcelo a day after receiving the JBC list, although some were predicting that the President might take longer to reach a decision.
Charges of possible conflict of interest stem from Marcelos connections with the Carpio, Cruz and Villaraza law office, the alleged preferred law office of the First Family.
Pimentel said he believes that Marcelo would follow his conscience in the prosecution of former President Joseph Estrada on plunder charges, and in any possible entanglement in the future with his former law office.
"I believe that he would use his conscience in performing his duty, and lets give him a chance to prove himself," Pimentel said.
In another forum, two civil groups, Plunderwatch and the Lawyers for an Ombudsman with Freedom and Tenacity (LOFTY), asked Marcelo to defer his oath-taking as Ombudsman.
Leaders of the two groups charged that the JBC, headed by Chief Justice Hilario Davide Jr., "railroaded" the selection process for the Ombudsman.
Dr. Carol Araullo, head of Plunderwatch, said Marcelo should defer taking his oath of office until all clouds of doubts surrounding the regularity of his nomination by the JBC and eventual appointment by the President are cleared.