The Yankees are plummeting where they should be thriving and thriving where they should be plummeting.

At the big-league level, the AL East title is in stunning jeopardy. A lead that was 15.5 games on July 8 — back in the innocent days of speculating whether we were watching the greatest team ever — is down to five after the Yankees lost two of three in Florida to the Rays. We can now speculate if we’re watching the greatest collapse ever.

The nosedive is unfathomable for several reasons, among them that this team should be better than it was a few months ago because of the help Brian Cashman acquired. Frankie Montas, despite his five one-hit, no-run innings Sunday, has disappointed thus far. Andrew Benintendi and Scott Effross are hurt.

But the talent pool that should be drained — the pitching prospects the Yankees consistently cash in when a proven, big-league piece can be dealt for — is humming.