KANSAS CITY, Mo. -- The Royals traded slugger Carlos Santana and nearly $4.3 million to the Seattle Mariners for right-handers Wyatt Mills and William Fleming on Monday, clearing the way for Kansas City to bring up hot prospect Vinnie Pasquantino.

The Royals optioned Mills, a 27-year-old veteran, to Triple-A Omaha while designating right-hander Ronald Bolaños for assignment. Fleming was assigned to Class-A Quad Cities.

Kansas City agreed to send $4,269,231 to Seattle, leaving the Mariners to pay just $1.5 million of the remainder of Santana's salary in the second year of a two-year, $17.5 million contract.

After hitting 19 homers in 158 games for Kansas City last season, and providing some veteran leadership to a team going through a long and difficult rebuild, the 36-year-old first baseman is hitting just .216 with four homers through 52 games this season.

This is the second time Santana has been with Seattle, though the first stint lasted a mere 10 days. He was acquired along with J.P. Crawford from Philadelphia for infielder Jean Segura, right-hander Juan Nicasio, and left-hander James Pazos on Dec. 3, 2018; the Mariners then traded him away as part of a three-team deal with Cleveland and Tampa Bay.

This time should be different as the 36-year-old gives Seattle a useful switch-hitter and an option at first base with leading hitter Ty France on the injured list with an elbow injury. Santana has been the primary first baseman for the Royals and has been better at the plate over the past month with a .357 batting averaging and a 1.032 OPS in June. Seattle also is receiving $4,269,231 from Kansas City to offset Santana's remaining contract, with $1.5 million paid by the Mariners.

With the Royals last in the AL Central at 26-45 heading into Monday night's game against Texas, and Santana nearing the end of his contract, it was prudent for Kansas City to clear the way for Pasquantino to begin his big league career.

The 24-year-old was picked in the 11th round of the 2019 first-year player draft out of Old Dominion and was generally one of the Royals' overlooked prospects until the past couple of seasons. He was hitting .280 with 18 homers at Omaha, and was among the Triple-A leaders in extra-base hits, runs, homers and slugging percentage.

The fact that they got a couple of promising pitchers in Mills and Fleming was a bonus.

Mills had a 4.15 ERA in eight appearances for Seattle this season, along with going 1-0 with a 1.83 ERA in 19⅔ innings for Triple-A Tacoma. Fleming was picked in the 11th round of last year's first-year player draft out of Wake Forest and was 6-6 with a 4.92 ERA in 14 starts for Class-A Modesto this season.

Bolaños had a 4.42 ERA in eight appearances for Kansas City this season.