For I am (for I also am, Revised Version) a man under authority, having soldiers under me (under myself soldiers, Revised Version): and I say to this man (this one, Revised Version), Go, and he goeth; and to another, Come, and he cometh; and to my servant, Do this, and he doeth it.
The centurion says that he knows the power of a command when given by one in authority, through the obedience that he himself shows and through that which he himself receives. Observe, he naturally orders his soldiers movement, and his slave work.
Further, may not "and to my slave" represent the climax of his faith? He felt that the powers of nature (at least those concerned in this illness) were not only subordinate to Jesus, but were completely under his power.
At his command they would act and the man be healed.