Gio Gonzalez registered almost a strikeout per inning last season, but also struggled with his control. (Ezra Shaw/GETTY IMAGES)

A year ago, Washington Nationals General Manager Mike Rizzo was prepared to trade four young players to the Kansas City Royals for former Cy Young winner Zack Greinke, the deal falling apart only because Greinke, with a full no-trade clause, refused the Nationals’ nine-figure contract offer in exchange for accepting the trade.

But the Nationals’ thirst for a front-line starter to join Stephen Strasburg and Jordan Zimmermann atop their rotation never lessened, and Thursday the team agreed in principle to a blockbuster deal for Oakland Athletics all-star lefty Gio Gonzalez in exchange for four of their best prospects. Keith Law of first reported details of the trade, which is pending physicals and likely won’t be announced officially until at least Friday.

In exchange for Gonzalez, a 26-year-old who has won 15-plus games and thrown 200-plus innings in each of the past two seasons, the Nationals are giving up three pitching prospects — right-handers Brad Peacock and A.J. Cole, and lefty Tommy Milone — plus catching prospect Derek Norris. Peacock, Cole and Norris ranked third, fourth and ninth, respectively, in Baseball America’s recent list of the Nationals’ top prospects.

The package was less than the one the Nationals were willing to send to the Royals for Greinke a year ago: That deal would have cost the Nationals at least one from a group that included Zimmermann, closer Drew Storen or second baseman Danny Espinosa, plus additional prospects.

At the same time, Gonzalez, a first-time all-star in 2011, is not without flaws. A power arm who ranked fourth in the American League in 2011 with 8.78 strikeouts per nine innings, he also led the league in walks, with 91, a year after finishing second with 90. It also appears he benefited from pitching in Oakland’s cavernous Coliseum the past four seasons, recording a career ERA of 3.56 at home versus 4.32 on the road.

The trade was the fourth of Gonzalez’s career since the Chicago White Sox drafted him as a sandwich-round pick (38th overall) in 2004.

Still, the Nationals, whose offseason to this point had been marked by high-profile misses and low-profile hits, suddenly have taken a major step in their quest to contend for a playoff spot as soon as 2012. With Strasburg, Zimmermann and Gonzalez atop their rotation, the Nationals have three young, controllable power arms that could form the nucleus of a formidable pitching staff for the next four-plus years. (Gonzalez doesn’t become arbitration-eligible until 2013 and is under team control until 2015.)

“Whatever team is willing to . . . put me in their rotation,” Gonzalez said Wednesday in an appearance on MLB Network Radio, regarding a possible trade, “I’d be more than happy to shine like a star there.”

The Nationals reportedly also are receiving a throw-in, pitching prospect Rob Gilliam, in the trade. A 24-year-old right-hander, Gilliam went 12-7 with a 5.04 ERA for high-Class A Stockton (Calif.) in 2011.

The trade, which cost the Nationals dearly in prospects but relatively little in dollars, is certain to renew speculation that the Nationals will make a run at free agent first baseman Prince Fielder, whose price tag could hit $25 million per year for seven or more years, in an all-out effort to contend next season with Philadelphia, Atlanta and Florida in the suddenly ultra-competitive NL East division.

None of the prospects the Nationals sent to the A’s in exchange for Gonzalez was considered irreplaceable. Of the four, Cole, a flamethrower who turns 20 next month and spent 2011 at low-Class A Hagerstown, is thought to have the highest ceiling, while Peacock, 23, stood the best chance of being a major contributor for the Nationals next season. He and Milone were expected to compete this spring for spots in the team’s 2012 rotation.

Called up at the end of 2011 after a breakthrough minor league season that included an appearance in the Futures Game, Peacock made two impressive starts and allowed only one earned run over the course of his 12 innings (0.75 ERA).

Milone, 24, is a soft-tossing lefty who also earned a September call-up after excelling at every step of the minors. His soft repertoire and impeccable command drew comparisons in the Nationals’ organization to a young Tom Glavine, but talent evaluators had mixed opinions as to whether he would be able to retire big league hitters consistently. A flyball pitcher, Milone likely will benefit from the Coliseum’s large dimensions.

The Nationals believe the recent influx of a new wave of pitching prospects into their farm system — including 2011 draftees Alex Meyer and Matt Purke, plus lefties Robbie Ray and Sammy Solis, both drafted in 2010 — gave them enough depth to part with a trio of more advanced prospects.

Meantime, the 22-year-old Norris, the Nationals’ top catching prospect, is an on-base machine (.403 career on-base percentage) who had seen his production slip in each of the past two minor league seasons, both of which were cut short by injuries. He also became expendable for the Nationals because of the emergence of 24-year-old Wilson Ramos as their No. 1 catcher.

Nationals note: The worst-kept secret of the Nationals’ offseason is about to reach its conclusion, as the team is believed to be close to reaching an agreement with free agent infielder Mark DeRosa.

DeRosa, 36, has a career batting average of .272 with a .341 on-base percentage and a .416 slugging percentage, but played in only 73 games combined in 2010 and 2011 for the San Francisco Giants because of a wrist injury.

DeRosa has played six positions in his career, but the Nationals apparently view him primarily as a backup at first base.