At the risk of boring some people here:
A subtheme of GFII is how Neri pushes past Rocco and attempts to push past Hagen to become Michael's Number Two man. In that penultimate boathouse scene, Rocco simply lurks in the background like the mere bodyguard he's become. Neri leads the discussion about Roth and his whereabouts. And, when Michael hurts Hagen by referring to his offer of employment and his mistress, you can tell from the smirk on Neri's face that he was the source of that info. The net of that humiliating exchange was: "Are you gonna come along with me in these things I have to do?" Hagen, humiliated, replies, "Why do you hurt me, Michael--I've always been loyal to you."
That sets the stage for Michael saying that "history proves that anyone can be killed." He then turns and says, "Rocco?" Notice that he didn't say, "Al?" The reason was that Rocco had become expendible at that point, but Neri wasn't expendible. And, at that point, Rocco, having witnessed Hagen's humiliation and all that stuff about loyalty, really had no choice but to say, "difficult, not impossible." He may also have agreed to do the job personally as a last-ditch attempt to prove that he was more loyal and more useful to Michael than Neri.