Wasn't it Clemenza who drew the Corleones' attention to Neri? ie: Neri was as much a Clemenza protégé as was Rocco

Sure thing Neri had already been picked for Fredo's killing So it is logical Rocco gets picked for Roth's Killing

from memory, among others -

• Rocco was the one who came over to the Church, to tell Michael that the Baptism massacre had gone swimmingly, to plan
• Rocco was the one with Michael at Hotel Washington, attending to important! papers

“Is it a boy?” scene even though Rocco was already in the Desert Inn room waiting when Michael, Neri and Tom walked in, Michael asks Neri to get him a wet towel. Michael also shakes Rocco's hand and pats him on the back

So I don't think we can read too much into such things – as to who is tasked to do what
I can't see any of Michael's men are expendable to Michael and Michael would not want to lose any of his men Let alone.... someone as established as Rocco

Difficult -- not impossible but Rocco still needed an escape plan among others -

• Michael committed a double murder including the New York Police captain in a public restaurant
• Neri killed Emilio Barzini including the driver, bodyguards, in public at the steps of the [court house?]

These murders were well planned, executed and the 'killers' got away. Rocco take note!