Perhaps Borges destroyed Bioy Casares! 
 The writing of it should have been a risky adventure. But the diligence those risks evoked in a young Bioy Casares diverted his mind from everything else. While it is a book of metaphysical speculation, the artifice that enables that speculation - the titular invention - is a greater obsession for the writer, so much so that the justification of that artifice becomes the center of the novel. (Contrast this with Kafka, where rationale of the artifices is dispensed with as soon as their concept is clear, and thereafter the artifice is only used, not described.) 
 I believe the book exists because of Bioy Casares friendship with Borges - it is dedicated to Borges. But shouldn't we posit now, now that we have the advantage of decades, that Borges may have spoiled Bioy Casares rather than made him? Clearly, here, in a bid to impress Borges by creating what Borges would term - in response to the work, though one can guess that it was a term he must have communicated to Bioy Casares often - 'works of reasoned imagination,' Bioy has bothered too much with the 'reasoned' part of it. 
 I'm tired. Because I had to read on or else I would have felt that discord that you feel when a heavily plotted book, one that may have a sublime revelation near the end, is abandoned. 
 Another perspective, one that will offer a different view to the novel, and one that may interest, will be presented tomorrow. 
 The alternate perspective 
 Borges, in his introduction, compares this novel with 'The Trial' by Kafka. Of the difference with Kafka I've mentioned a small point earlier. But if we are to assume that Borges was pointing to the greatness of this novel, we need to look for that greatness not just in comparison to Kafka but what this novel does in itself. In my view, there is an interpretation that allows for greatness. 
 'The Invention of Morel' might be a work that 'novelizes' the desire of the narrator unlike anything before. To elaborate that statement I'll have to go into spoiling, just a bit. The unnamed narrator is on an island where he sees some action and describes it for the benefit of us readers. But just as in a book, that action has limits, after which it can only be revisited. In fact, this repetition can go on till eternity, and for it to remain possible to be narrated to us, the only condition is that there be an eternal narrator. In Bioy Casares's work, this notion of the eternal narrator who is at your service as soon as you open the pages is brought to question. The narrator wants to be a part of the fiction - rather, the invention. The narrator has a consciousness, and an ambition to be in turn narrated. 'The Invention of Morel' is basically an invention that allows for narrators to merge into the fiction.