KANSAS CITY, Mo. — The moment Alex Gordon knew the Kansas City Royals were serious about winning can be traced to a December day when his wife heard they had traded for James Shields.

The franchise had long suffered through a forgettable cast of starting pitchers. Hot prospects flamed out. Free agents fizzled. And every year, the Royals languished near the AL Central cellar.

But things changed in December 2012. General Manager Dayton Moore thought enough pieces had been assembled and all that was missing was the right starting pitcher — someone who could not only be the staff ace but who could change a clubhouse culture accustomed to losing.

Moore called up the Rays and made the deal.

“That’s when I knew,” Gordon said, “that we were going for it.”

Two years later, a trade that was panned by many has helped the Royals reach the World Series. Shields, the presumptive Game 1 starter Tuesday night against San Francisco, has been everything Moore had hoped he would be.

“Those opportunities to acquire a top (of the) rotation starter and an impact pitcher like Wade Davis, they’re not presented year-in and year-out,” Moore said. “We were fortunate the timing of it was such that it was staring us in the face and put us in a position to compete in 2014.”

It was a gamble. The Royals sent baseball’s top minor league talent, Wil Myers, and a bevy of other promising prospects to the Rays. They were mortgaging their future to win in the present.

The trade paid immediate dividends. Shields went 13-9 with a 3.15 ERA last year, helping the Royals to their best record in more than 20 years. And over the course of the season, Davis established himself as one of the most dominant late-inning relievers in the game.

This year, Shields is 14-8 with a 3.21 ERA, helping the Royals not only return to the postseason for the first time in 29 years but advance all the way to the Fall Classic.

He started their wild-card win over Oakland. He allowed two runs over six innings in earning the win in a divisional game against the Angels. And he was good enough to help the Royals knock off Baltimore in the ALCS, running their postseason winning streak to 11 games.

“He’s earned the nickname ‘Big Game James’ for a reason,” Royals manager Ned Yost said.

On a team with precious few veterans, Shields has proved invaluable in October. And when he takes the mound against the Giants, he’ll be drawing on the experience he gained in 2008, when he tossed 5.2 innings for the Rays against the Phillies in the only other World Series start of his nine-year big league career.

“He’s been tremendous,” said Greg Holland, the Royals’ All-Star closer. “He takes that starting five as kind of collectively, ‘Hey, we want to be the backbone of this team.’ We want to throw 200 innings apiece. We want to push each other, learn from each other.

“I think he also leads by example, taking the ball every five days and giving his heart and soul. I think once you start rattling off good start after good start, it kind of builds on itself. You want to do just as good as the guy in front of you did.”