It doesn't have to be Belt, I'm just not sure who the obvious alternatives are.
Neither do I.
As you said though, we really don't know what's going on with Casas. Maybe this is one of those things where they'll have better information in the near future. If he's out for 5 weeks, anyone who has to ramp up is not likely to help. If he's out for 9 weeks, it's a different story.
I think the main problem is that there aren't a lot of starting MLB 1B just hanging around out there without significant problems in their bat or glove. Otherwise they'd be DHing at the ML level, or they're AAAA journeymen types. And we kinda got one right now in Dalbec.
They could trade for a AAAA type for defense and hope the bat clicks. But we kinda got one right now in Dalbec.
They could trade for a MLB bat-first guy and put his warm-body at 1B and hope he does not yip the middle-infield into thinking they have to make perfect throws.
They could trade for a AAA flawed 1B type with a maybe-bat and hope they get hot/lucky. But we kinda got one right now in Kavadas.
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But I don't see them trading significant assets for a true 1B type. Or a dedicated corner IF with a legitimate bat, because there's the Dalbec/Devers/Yoshida dynamic to deal with in (hopefully) a month or two. But maybe they'll grab a Josh Bell or someone.