That's kind of the thing about this matchup. McGregor could have the best straight left known to man, and it's still only one punch. From what I've seen he doesn't really have a jab or much of a hook, doesn't throw well to the body. If Floyd only needs to worry about negating one thing, no matter how good that thing is, he's going to have an easier night than usual.
I've always thought of fighting Floyd as trying to do one of those Simon-says light puzzles. You don't beat him by making the one green light go off over and over again. You beat him by throwing so many different looks at him that he'll eventually make a mistake. That's why I think guys like Cotto, Maidana and Castillo had such great success. A simple thing like changing your level between punches gets you one step closer to Floyd messing up the rhythm. Eventually he's got to start taking chances to mount his offense, and that's when someone like Cotto catches him with a jab or hook from below; it's when Maidana catches him with a counter that stiffens him up. It's all built around variety.
At the end of the day though, it's just really hard to do.