I don't think we're gonna send him down. I'm not entirely sure how that'll work, but he looks like he's earned Cooper's trust, and that's not the easiest thing in the world. I kind of think he's going to replace Girardi with Hedman. I just have no idea how we're going to make it work.
I don't see why we would shoot ourselves in the foot and not send him down when healthy. The next option is trade KK because that guarantees no more playing time for him. Like Macho said going back down doesn't hurt Cernak one bit, we're in massive cap crunch next season and will need every good contract you can get, and Koekkoek is just that. He has looked for the most part very serviceable on the bottom pairing and showed bright spots in his brief appearance this season.
The goalies haven't been great but the defense has also performed poorly since Stralman went down, if we're bringing Stralman back that's all the more need for cheap servicable defenseman. We benefit more giving icetime to Koekkoek right now and fully evaluating him over the marginal benefit you get from Cenerak as opposed to Girardi, Cernak is already a given for the next season. If you don't bring Stralman back you dip into either free agency, trade for someone elses cheap unwanted defenseman or roll 3 rookies and a developing Sergachev.
The best option is to give KK more playing time right now, if he blows then we can scratch/trade him and call up Cernak again but either way he'll likely be going for at least one more trip back down.