I don't think that Shane won't be a problem for Mayweather, but not exactly for the reasons you've mentioned, and I don't think the reasons why Mosley had problems with other fighters is why Mayweather will trouble him so much.
First of all Mayweather is a much purer boxer than anyone Mosley has faced, even Winky Wright. Mosley has never experienced a guy who moves and boxes as well as Floyd. Second Floyd adjusts better than anyone Shane has been in there with. It's not just that Floyd's opponents tire in the late rounds because almost everyone does, but Floyd makes adjustments and he stays fresher than anyone else. I think this will remain true against Mosley who doesn't usually appear really sharp in the first place (I mean by this that he gets hit by shots early on as he does late). Also despite Floyd being smaller than Shane I think his style does pose similar issues that facing Vernon Forest and Winky WRight did for Shane. Though Floyd is smaller than Shane he is a more effective fighter on the outside. I think this gives Shane problems because as good as his chin is, he doesn't like to just walk in impose himself physically on guys which I think he could have even done to the bigger Wright and Forest, but also because he doesn't usually fully comit to coming forward we saw him being hit quite easily as he hesitated at the wrong moment. I think Shane did a better job of that against Collazo, but I honestly think that was only once Collazo hurt his left hand badly.
I do think Shane's physical abilities and his physical abilities alone will let him be competitive in this fight. I just have a hard time seeing him get the oppertunities to be effective simply because he doesn't have great outside game. Also one more problem I have always had with Shane is that he doesn't fight at the speed of his body. That's why I think we saw Winky beat him to the punch, Forest beat him to the punch, and Cotto beating him to the punch. I think Mosley moves incredibly fast and even has fast reflexes, but his timing is off and his intuitiveness defensively seems to be lacking for a guy of his talent, and maybe its because he has a choppy transition from defensive to offensive and vice virsa.
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