Not really true. Manny was fine even thru the Marquez fights, JMM was just a guy who always fought Manny tough and who was clearly roided in that fight.
Also the version of Floyd that Manny fought was just as much not in his prime anymore either. Floyd's movement greatly declined around the time of the 1st Maidana fight. The result of a Floyd/Manny fight would have been the same regardless of when they fought. Manny would have had a much better chance if they'd hypothetically been able to fight at 135 or 140. Pac's resume at 147 is mostly fluff, fighting guys who were either weight drained or clearly past their prime (or both in the DLH fight).