I haven't read Cobble's book (and have never heard of her), but just because Job 39.9 mentions a manger doesn't mean it's a prophecy of Jesus.

Let's look at the context. Job 38.1-42.6 is God's conversation with Job at the end of the book. Instead of staying remote (transcendent), God actually shows up to comment on Job's "legal case" against Him. God claims that Job has too little understanding to understand how God runs the world. His answer suggests the idea that God runs the world by the mysteries of His wisdom, and can't be interpreted by human suffering or held to account by the retribution principle.

In Job 39, God is showing us a picture of the animals He created and loves, and that Job can't even understand the animals that are in front of his eyes, known in his environment. That's the context.

In vv. 9-12, he gives us a picture of the undomesticated and untamable wild ox. Will he consent to serve you? (A "no" answer is implied.) Will it stay in your barn and await work the next day? (no) Will he allow you to harness him for work the next day? (no). Will he create furrows at your guidance? (no) Etc.

There is absolutely no reason to view this as a prophecy of Christ.

I have heard from one source in particular (Tara-Leigh Cobble) that Job 39:9 is a possible prophecy regarding Jesus, but I’m not able to find that argument being made in my study bibles or in commentaries I looked for online.

Job 39:9 “Is the wild ox (translated as unicorn in KJV) willing to serve you? Will he spend the night at your manger?”