After reaching Game 6 of the AL Championship Series last season, the Mets are looking to improve their roster.

The ongoing rumor is that the Mets are going all-in on the biggest free-agent prize, Juan Soto. But beyond the left-handed slugger, they have other important holes to fix, especially at starting pitching.

The Mets have already filled one of possibly three vacant spots in the rotation with Frankie Montas. How else do they intend to fill out the rest of the starting five?

On Tuesday, ESPN senior writer David Schoenfield suggested the Mets pursue the trade market to improve their rotation. In particular, Schoenfield proposes they target three-time All-Star right-hander Luis Castillo from the Seattle Mariners.

“They could go to the high-rent district and sign Corbin Burnes or Max Fried,” Schoenfield writes, “but maybe a trade makes more sense, as Castillo, with three years of team control, is a safer bet than signing a player to a huge long-term deal.”

What would it take to acquire the likes of Castillo?

“The Mariners could roll the dice on Brett Baty — who doesn’t have a spot on the Mets — to fill third base, with another prospect or two included as well,” Schoenfield adds.

Here’s why a Castillo trade to the Mets could work:

Castillo signed a five-year, $108 million contract with the Mariners ahead of the 2023 season. He is expected to earn $24.15 million guaranteed over the next three seasons and has an option that vests for 2028 if he pitches at least 180 innings in 2027.

Castillo has reached the 180-inning threshold three times in his career. Two of those occurred while he was with the Cincinnati Reds in 2019 and 2021.

His contract is currently tied for the 14th largest among starters in MLB. On an average annual value basis, it’s worth less than what the Arizona Diamondbacks pay Jordan Montgomery and what the Yankees pay Carlos Rodon, among others.

Castillo would cost something similar to what comparable free agents are expected to earn.

Right-hander Nathan Eovaldi had a higher ERA (3.80) than Castillo (3.64), but his market value currently exceeds Castillo’s. Eovaldi could get a contract with a $21.8 million AAV — Castillo’s contract has an AAV of $21.6 million.

Left-hander Max Fried was virtually identical to Castillo in most pitching categories — he bested him in ERA 3.25 to 3.64 — but is expected to get a long-term deal with a potentially higher AAV than Castillo.

The same goes for Jack Flaherty, who nearly cost the Dodgers Game 5 of the World Series had it not been for an epic Yankees meltdown. Flaherty could get a similar contract to what Castillo is owed, but the latter has a better track record, making him a more attractive option.

A rotation anchored around Kodai Senga, David Peterson and Castillo would be tough to beat. The Mets could also still be in the market to re-sign Sean Manaea or Luis Severino, making them one of the best rotations in the National League.