Currently Buffalo, with games in hand, is 6th from the back. Horvat will help that, and I have to imagine the Sabres would do better anyway, but that very easily could bump the pick until next year, or to a second, or to a lower pick acquired later, or what ever the protection dictates. Next year doesn't have the hype around it that this year does and a lower pick (a later first or a second) doesn't really pop either.
A high second (or the highest of three possible) is fine as a tertiary piece, agreed.
Rosen is a tiny (in terms of weight) winger, which we don't need. Yes, yes, any prospect could help, but comparing him to Lambert or Heinola from Winnipeg, or Newhook from Colorado, or Lysell, or Niemela, or any number of Kings or Hurricanes or Wild or Capitals prospects, or so many other prospects that have been thrown around, Rosen is nothing special. We also have our two top prospects, and two top young players, playing wing in Klimovich and Lekkerimaki, and Podkolzin and Hoglander, respectively.
Considering his (unsustainable) trajectory and what past rentals have gone for, I don't think this is an insane haul. Chiarot got a first, a third and a prospect last year. Copp got a first, a second, Barron and a 5th last year. Giroux got a first, a third and Tippett. Lindholm got a lesser prospect, a first and two seconds. Giordano got a third and two seconds. Kuemper got a first, a third and Timmins. All of these guys were rentals at the time of acquisition. It might be a bad trade for Buffalo, but from a Canucks standpoint its very bland.
Myers being involved is also going to affect the return, I'd argue in our favor, especially at half retained for both players.