“I fear a trick,” says Thetis, after telling Achilles that “the best of the Myrmidons” will die before he does. (Which is exactly what Briseis calls Patroclus when he sees her for the last time.)
And then, when Patroclus tells Achilles to send him out in his armor: “the words shocked us both. They seemed to come through me, not from me, as though spoken straight from a god’s mouth.”
I think this line, about fearing a trick, does a lot of heavy lifting.
One: Thetis is pretty clearly foreshadowing here: Patroclus tricks everyone into thinking he is Achilles on the battle field.
Two, as Patroclus explains, this is the Fates doing what they do, telling the future but withholding the full extent of what they mean. It won’t really be clear until it all unfolds. The best of the Myrmidons is not Achilles, it’s Patroclus. Thanks to the own gods’ trickery and intervention in the war, they manipulate what’s happening to the point that Patroclus is divinely inspired to go out and play the part he was destined for, too.
But wait - there’s more! Thetis is the first person (ahem, goddess), who would list Patroclus last possible option for the role “best of the Myrmidons.” Thetis is not able to understand this piece of the prophecy because she’s not yet able to see Patroclus for the hero he is, too. The Fates are tricking Thetis right to her face. Maybe this could’ve been the moment, hearing this prophecy, to have a change of heart and realize what her son would be up against. But she didn’t. And yet, despite her dismissal of Patroclus’ death, and despite her walking away from Achilles when he rails against her for it, by the end of the book Thetis has come around. She too finally realizes that Patroclus was the best of the Myrmidons. And she gives him what he deserves, at last.