Sabres haven't given up on Mitts yet. I doubt they move him unless it is for immediate help at 2C.

Buffalo passes

I don't see anyone giving you a 2C for an under-performing prospect. Still, it is way to early to sell on him unless a team overpays. It is comparable to Patrick for us. We lucked into the pick and have to hope he can return to the ice and play at a decent level.

I saw this article about Mittelstadt yesterday. Main quote below:
"Casey Mittelstadt isn’t a product of a bad situation in Buffalo. The Sabres simply got fleeced. As the NHLe model shows, Mittelstadt had no business being featured in the top-10 of the 2017 Draft rankings. "

Noone is expecting to get an established 2C for Mitts straight up. Just like I would hope that Flyers fans would not expect an established top 6 player for Nolan Patrick. We still have Hope's that he can become a 40 - 50 point middle six forward or possibly a slim chance he becomes the high end 60 -70 point 2C he was drafted to be. He would be a piece in a deal for a player that has already proven they can be that #2 center which we desperately need. It would appear that he has already outperformed this site's projection as only about 15% of third round nhl selections become low level or better nhl players where they had him projected. It appears that Mitts will at least be that even if he does not become the player he was drafted to be.

https://www.capfriendly.com/forums/thread/341165?post_id=1549124
My comment in that post:

"First of all, any article that quotes Chad DeDominicis has to be viewed suspiciously. No knowledgeable Sabre follower takes him seriously.
Second, I would never say Mittelstadt was a lock for #2 center, in fact, I would have sent him to Rochester last year after watching him perform in the Sabre's Prospect Challenge.
Third, he is hardly a bust, with 39 pts in 114 NHL games as a 20 year old.
Fourth, the article was correct, it was a terrible draft which should have automatically meant he should not have been brought into the NHL as soon as he was.
Fifth, while he did get PP time, he made the most of it getting more goals than Sam and was ranked 5th among all Sabre forwards with 10 PP pts. Let me add, he only averaged 13:27min/game in 2018-'19 which was less than Larsson or Girgensons. So to say he got mid six minutes would be a gross exaggeration.
Sixth, he needs to get bigger just as Sam Reinhart did. Once he does that and his ceiling is only 40pt, then I would be disappointed."