I hesitate to comment on autographs since there are much bigger experts on here than I am.
That said, since no one has has commented I'll share my opinion that I think it is legit for a few reasons:
1) It looks basically identical to some other rushed Mattingly signatures that I have seen online. Everything from the little strange loop on the top left of the D through the end of the signature, including the angle of the slant (which is appears is something that he was not super consistent with over time, but matches to others that look a lot like this one), the funny loops at the end etc. looks a lot like many other one you can find on Google that have been authenticated.
2) Signature looks very fluid, like it was written quickly. One of the common signs of a forgery is that the signature appears to have been written slowly and laboriously as the forger tried to get it just right.
3) Mattingly is a relatively cheap signature, so likely less of a target for forgers. This alone is no guarantee, but it seems unlikely someone who could forge a signature almost perfectly would spend the time working on forging Mattingly, instead of players who would sell for a lot more.
Again, not an expert, so take it for what it's worth...
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