Throat of Fireplace Cracked - Danger?

I have a zero-clearance wood-burning fireplace that has a throat that is deteriorating and breaking. The throat damper was removed and replaced with a chimney top cap/damper before I bought the home because it was so rusted it wouldn't open. The throat (if this is what it's called) is a metal housing that is located between the beginning of the chimney and the body of the fireplace (the hearth?) where the throat damper used to open. It is shaped like a squared upside-down funnel, with a ledge toward the back. There sits debris on this ledge that looks like rock and stuff, residing there from the building of the chimney, I suppose. Looking further up the chimney, it appears that there is a metal lining going all the way up too.
Just above the ledge, parts of this metal throat have chipped away from becoming so rusty, and exposed the brick behind it. There is a small hole in it, about a 2" diameter. No fire ever gets up to the ledge, but flames do sometimes rise near where the throat starts. I'm concerned that either smoke or creosote could be penetrating behind the housing, into the brick layer behind the throat, causing some danger. Should I be concerned about this and call the chimney expert, or is this something that will not affect the safety of the fireplace?
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should expose nothing but fire-rated brick/tile or metal. Regular brick/mortar is porous, and creosote can migrate through. Saw a lot of that in ancient houses, where it leaks through and stains wallpaper in upstairs rooms. While unlikely, if enough gunk build up back there, you could get a fire outside the liner. More important, it could also be leaking CO into any spaces the chimney passes through.
Get a estimate or three from local masons specializing in fireplace reconstruction. They may be able to do a reline reasonably cheaply, either with metal, or an inflatable tube they pump a special concrete mix around. Worst-case scenario- they open up the front of the fireplace in your living room, and graft a new box in there.
aem sends....
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