No reason not to make this happen IMO. The slot is already on the card, right? If Magny says he's ready, in shape, and willing, then let him slide in and shoot his shot. They can still have the Leon fight in the new year as planned if/when Borz wins, which will be all the more justifiable if he knocks off the #9 contender beforehand. This gives Leon time to recover and Chimaev prides himself on staying active anyway, no?
Furthermore, Magny post-layoff and Leon are stylistically similar in many ways. Khamzat could get some very valuable experience and similar looks fighting the Haitian Sensation and carry that into his fight against Rocky.
Just my two cents.