Lazio coach Stefano Pioli feels the referee got it wrong in the Rome derby, claiming Roma should not have been awarded a first-half penalty.
Edin Dzeko's early spot-kick and Gervinho's strike after the break condemned Lazio to a 2-0 defeat and their third successive loss in Serie A on Sunday.
Dzeko's 10th-minute penalty at the Stadio Olimpico was controversial, referee Paolo Tagliavento pointing to the spot despite Lazio defender Santiago Gentiletti appearing to have made contact outside the area.
"Today we weren't short of drive, but the game started out on the wrong foot, with a nonexistent penalty," Pioli said.
"There can be no doubt that the contact took place outside the box. I'll say this again: there was no penalty.
"Episodes like these are decisive. There were many people who could have seen the situation, even though it was difficult for the referee from his position.
"But if he wasn't certain, it would have been best to take a different decision."
Lazio dropped to ninth in the standings, nine points adrift of joint leaders Inter and Fiorentina following the derby loss.
Pioli added: "The third defeat in a row worries me. The derby was an important game. Emerging without a positive result makes things more difficult.
"Now we just have to keep our heads low and charge forward to get out of this situation."
