Well to be fair, I think it was Manny that suggested to Freddie that they need a knockout this fight, rather than the other way around. And by Roach agreeing, I think it's because he knows that getting a wide decision on Marquez in Pacquiao's favor is going to be nearly impossible....Marquez is just too good. Plus, I think Freddie is worried about the judges being extra critical of Manny after the controversial decision of their last fight.
IMO, I don't think Manny necessarily NEEDS a KO over Marquez. He could win a decision if the fight is clearly one-sided (highly unlikely). If it's close and it goes to the judges, I can see the judges giving it to Marquez this time. So I can understand why both Manny and Roach are looking for the KO.