The chief problem for the Ukrainians what I see, is that they don't really have units to counterattack with to halt Russian advance then stabilize the front.
It seems the Russians attacked between a territorial defense brigade and a mechanized one. And then they have an ongoing offensive both to the north and south, at Kupiansk, and Kreminna. So Ukrainian units there are tied down with the defense and they might not have units to spare. They'll send some for sure. Because they have to some. But any major reinforcement has to come elsewhere.
AFU has uncommitted brigades which was prepared for the spring/summer offensive. Sending them could be an answer. They can be there relatively quick, but large columns on the move can be struck, and Russia has the tools to do that. Their route have to be covered as much as possible, which might distract units from other tasks.

That is if this deployment map is reliable at all.