From the "pro-Russian" perspective, you could spin this as setting things up for a ceasefire/peace settlement. The Ukrainian recovery of that territory enables them to claim enough success to negotiate a settlement over other territories. I'm not at all convinced that's Ukraine's intention, but Russia could easily afford to lose the territory north/west of the Dnieper if that was the price for keeping everything else.
The Ukraine government could negotiate anything - but the people will stand for nothing less then the whole of Ukraine.