User blog comment:Gourgeist/Shingeki no Kyojin Chapter 65 Review: Dreams and Curses/@comment-7418318-20150112232657/@comment-7418318-20150113123050

Oh right... fluid not marrow.

I didn't really mean donate like we understand it today. They could literally rip out the lower part of his spine and let her eat it directly (in Titan form)... well not exactly ripping, since that would also damage main arteries and cause him to bleed to death, but you get the idea.

If Eren's regenerative abilities kick in, all is fine and if not, he'll have paralized legs for the rest of his life, but at least he'll still be alive and quite possibly be able to tutor Histoira about being a Shifter. It's a bit of a gamble, but absolutely worth it. If the gamble doesn't work out, they'll have lost nothing compared to killing him and if it works, everyone wins, except for Historia's father, but screw him.