Manny Pacquiao’s reputation as the top pound-for-pound fighter in the world has taken a hit over the past few years. The Filipino lost a controversial decision to Timothy Bradley in June, and he narrowly won a majority decision over Juan Manuel Marquez in his previous fight. Though he battered three straight opponents before that, he hasn’t scored a knockout since 2009. Pacquiao’s trainer, Freddie Roach, recognizes that Pac Man needs to bring back his killer instinct to regain his reputation as a fearsome fighter in the ring. “Marquez will be more hittable and not so much a counter-puncher. I think we need a knockout to secure [Pacquiao's] No. 1 spot in the world. He needs to be impressive and a knockout would be impressive,” Roach told Boxing Scene.“In the first [fight with Marquez, Pacquiao] was aggressive in the beginning of that fight. In the second fight we had one knockdown and in the third fight we had none. He just lost that killer instinct. In the first fight he was trying to knock him out, but Marquez is very resilient also.“You have to take that factor in also. We need to bring back that killer instinct where he finishes guys like he used to.”Pacquiao and Marquez fought to a draw in their first fight, and Pac Man won decisions in the next two. “[Marquez] lost those fights and I still had Manny winning by a little bit. Manny started well and he finished well,” Roach said. “A lot of fans thought that we lost [the third] fight so we have to redeem ourselves.”The fourth — and hopefully final — fight between the two is on Dec. 8 in Las Vegas. Despite what Roach is aiming for, Marquez seems to have Pacquiao’s number. I don’t see a knockout for Manny in that one.