Mrtka at this point seems more likely to go to the AHL. And yes, apparently he can go to AHL - I saw in another thread that the reasoning had to do with him being on loan from his Czech league to the WHL, rather than a typical North American player, not sure if that's actually what's happening, but several articles have been written about the likelihood of him going directly to the AHL.
https://thehockeynews.com/nhl/buffalo-sabres/latest-news/mrtka-s-future-may-not-include-return-to-whl