Exactly, and Clemenza actually makes this point explicitly when he tell Mike that he is considered to be a civilian. Sollozzo reckons Mike to be a "nice college boy" who would blanche at the very idea of holding a gun, never mind firing one at a New York police captain.

Sollozzo was in a weak position; he had tried and failed to kill the Don. He therefore had to negotiate, but realized that if he showed one hair on his head, Sonny's buttonmen would take him out. Who better to talk to, then, than the Don's college boy youngest son, a decorated war hero, with no involvement in the family business? Who Carlo probably told him was, if anything, opposed to the Don's line of work?

Mike was the only man who could have gotten to Sollozzo simply because the nervous and paranoid Sollozzo was on high alert following the failed assassination. A "civilian" was required, and Mike stepped up.