At this point, I think the only move that makes any sense is to get rid of one of Kindl or Lashoff and then bring up Marchenko (or Sproul, if he can show he's ready). The answer here has to be to go a different direction and play a kid. That's the way to turn this negative into a net positive, because these kids do have lots of potential. Stockpiling underachieving veterans on the lower pairings will prove to be a shortcut to Jack Eichel town. Although, now that I mention it, that doesn't sound so bad...
It might sound awesome to him to have 5 amazing defensemen in GR, but there just isn't enough ice. It's not good for somebody like Jensen to play 15 minutes on the 3rd pairing. and somebody like Marchenko doesn't have a lot to learn there, anyway. He's exactly the type who can learn on the job. That's what he had to do in the KHL, and he did a pretty good job of it then, too.
Plus, given the choice between Lashoff/Kindl and Marchenko, who shoots right, Babcock is going to pick Marchenko. So we don't have to worry about the guy being buried.
Why I think this could actually happen: they have Marchenko at the development camp. No other GR regular is going. I'm pretty sure the reason for that is to get him rehabbing his ankle very early on. The organization knows they need him ready to go for main camp. I don't think they'd push so hard if he was just going back to GR.