A major subtheme of GFII is how Neri tries to push past Rocco and Tom to become Michael's Number Two man. You see it in many places: in Anthony's party, when Rocco is little more than a rent-a-cop; when Michael introduces Neri to Ola while dispatching Roccco to get food for Johnny's men; when Rocco, not Neri, shuts up Deanna; when Rocco, not Neri, responds to the Tahoe shooting. And, in that boathouse scene, notice that Neri's sitting, leaning back, expounding on Roth's chances, while Rocco remains standing, saying nothing.
Rocco was Clemenza's protege; Neri was Michael's. Rocco had become expendible by the end of II. After Michael humiliated Tom by claiming that anyone could be killed, he turned and said, "Rocco?" Notice that he didn't say, "Al?" He needed Neri, he didn't need Rocco.