I don't think it was so much a matter of Rocco being 'expendable' as him being the best man to accomplish the job. Logic would dictate that had this been a less difficult and/or less important hit, it could've easily been handed down to reasonably experienced buttonman. However Rocco had to assure Michael that this would be done.

As for being 'expendable', ... this was the business they had chosen. Long term loyalty aside, you literally put your life on the line whenever you did a job for your Don.

Apple