One of the subthemes in GFII is how Neri tries to push past Rocco and Hagen to be Michael's #2 man. At Anthony's party, Rocco's like a rent-a-cop while Neri's in the boathouse waiting for Johnny Ola. Then Rocco's a go-fer: Michael dispatches him to feed Johnny Ola's men while he and Neri meet with Johnny (Rocco joins later). Rocco, not Neri, gets the s**t job of dragging drunken Deanna off the dance floor. After the attack, Rocco limps around trying to catch the attackers; Neri's probably sleeping. In the boathouse scene near the end, Rocco just stands mute while Neri, reclining in a chair, leads the discussion. And notice from Neri's sly look that he was the one who leaked to Michael about Tom's mistress and his employemen offer.
But, in the most revealing part of the scene: Michael says he wants Roth's plane "met." Hagen immediately says it's impossible to kill him in the airport. Michael humiliates Tom by saying that if history has taught anything, it's that anyone can be killed. Then he instantly turns and says, "Rocco?" Notice that he doesn't say "Al?" In effect, he gave Rocco no choice: either Rocco took on this likely suicidal task, or he'd be in the same doghouse as Hagen.

I believe that, by that time, Michael concluded that he didn't need two bodyguards. Rocco was expendible, Neri was more valuable. That's why he chose Rocco for this one-way mission.