On the surface, Monygham is referring to the mission to Cayta, but the novel is not speaking "plainly" by any means! There have been two key references to Nostromo as a "man," the first in which he proclaimed his physical splendor , the second a comment upon his awakening into disillusionment. Here there is a sense that Nostromo is "the only man" because he is the common People, the only ones willing to do the work of society, the only ones who really hold it together. In this sense, and in this context of nascent class-consciousness, the line brings together both his physical power and his awakened disillusionment.