I would be leery of using the brittle-looking muslin as is. It probably won't hold up as well as the rest of the quilt. I'd consider one of 2 things. 1) Carefully remove the Sues from the muslin and reapply them to a larger piece of the new muslin when you get it. or 2) Back the brittle blocks with the very thinnest iron-on pelon you can find. I've done this and it worked fine, then cut the other blocks to size.