This feels like a good deal for both teams given the context.
Vegas gets the gamebreaker centre they need to finally put them over the top (last year's playoffs reinforced this weakness). They only sacrifice depth - which they've had a surplus of since joining the league, and should continue to depending on who's dealt to make the cap work. Obviously if the injury has long-term impacts, it's bad...but even then, they haven't bet the farm as the team should still be loaded for several years.
Buffalo does okay by getting a good complementary, cap-controlled player (Tuch), potential top line-ish player (Krebs), and some reasonably valuable draft capital. That's not a great return for a top-10 talent...but under the circumstances it feels reasonable...especially if you never believed the rumoured deals like Calgary's were real. (I don't.)
The only issue I see from Buffalo's side is that the circumstances which hurt Eichel's value were largely of their own creation. I don't think they could have done better now...but I do think there were better deals to be had in the summer before the relationship went totally south and the surgery delayed. Management still wears that.