CLEVELAND — You know what should never be said about the Mets’ trade for Robinson Cano and Edwin Diaz?

That it can’t get worse.

Cano and Diaz have sunk well below the worst predictions for their seasons. Cano is one of just five players with at least 250 plate appearances, four or fewer homers and an OPS below .650. Diaz has an ERA 26% below MLB average factoring in league and parks, fourth worst among those who have at least 35 relief appearances.

Jay Bruce and Anthony Swarzak were traded back into the NL East, where they not only have performed well for the Phillies and Braves, respectively, but been Met killers.

Justin Dunn and Jarred Kelenic were invited to the Futures Game, marking them among the majors’ best prospects, with Kelenic particularly climbing prospect lists.

And then this came Tuesday from Commissioner Rob Manfred: “It is a 2021 issue.” That was in regards to the DH being adopted to the NL as part of collective bargaining agreement negotiations with the union. Thus, the earliest it would be introduced is 2022, and Manfred said, “I do not think it is inevitable” that the DH is coming to the NL.

(That could be a negotiating ploy, since the Players Association wants the DH in the NL, and Manfred termed it “a major concession” if the DH became used in both leagues.)

So what does this have to do with the Mets trade?

At the time it was finalized, the Mets’ belief that the DH was coming soon was part of their justification for taking on the final five years of Cano’s contract, his age-36-40 seasons. They deemed even if Cano aged out of playing second base, they could always install him at DH. But already he is aging out of second and there is no place to park him even if his bat justified the plate appearances.

The earliest would be 2022, when Cano will be 39 — and that also is two years after the contract expires on Yoenis Cespedes and his surgically repaired heels and ankle.

Maybe Diaz will perform better and Cano will hit closer to his career norms. But what if Dunn becomes a steady No. 3 starter and Kelenic a star outfielder?

It can always get worse.