No, we still have Austin Davis, who has shown to be at least a decent starting option. If Manziel cannot show that he deserves the starting job over another young and somewhat proven QB, then you don't start him, but I think you still start the next off season with Manziel as the #1.
Fact is, nothing we get for trading Manziel, or Gordon for the matter, will be worth the risk of trading him. If we could recoup a 2nd rounder for each, then maybe, but anything other than that you should just let their contracts play out and see what you get.
I haven't seen anyone so far in this draft outside of maybe the Mepheis kid who would be worth drafting and giving up on Manziel. Hopefully some of those preseason "1st round" Big Ten QB's fall down to the 3rd round or later, I wouldn't mind picking up a guy to bring in and test out.