It will be at Welterweight, i wouldn't say that he lost his power. The fighters he faced recently have got a decent set of whiskers and are top fighters. What you don't grasp is that styles make fights, Marquez is Mannys bogeyman but don't forget Manny was giving Marquez a hiding and was on the verge of a stoppage before being caught with that perfect right hand, that's boxing.
And just take a look at the body change of Marquez since he lost to Mayweather, he all of a sudden became huge and suffered from acne all over his back..... Very strange indeed....