I wouldn't trust him. I suspect the Leafs will add to their defence and all of this will be a moot point, but I don't think he should be anything other than a Plan B for the Maple Leafs in the playoffs.
That being said, I like this deal for them, even if Gus wasn't included at all. Flipping Sandin for a 1st rounder, even a late first rounder, is a win for the Leafs.
Sandin was often over-rated around here. He was a big factor in the Leafs losing against the Canadiens because they exploited him because he played scared, was easily rattled and mades mistakes. I didn't see the growth I would have liked to see out of him since then, either.
I don't think they wanted him to see the ice in their best case scenario in the playoffs and if you can't trust him to play in those games then I think it was wise to cut bait while they could still get a 1st round pick for him.