Edwin Jackson and the Nationals agree on a one-year, $10 million deal. (Dilip Vishwanat/GETTY IMAGES)

The Washington Nationals and free agent starting pitcher Edwin Jackson agreed Thursday on a one-year, $10 million contract, providing the Nationals a fire-balling right-hander to slot into a formidable — and now crowded – starting rotation.

Jackson, 28, gives the Nationals at least seven qualified starters. While the Nationals could ultimately look to trade John Lannan or Ross Detwiler, they do not plan to deal either before the start of spring training, a Nationals official said. The official also described talk of Lannan being shopped as “overblown.”

General Manager Mike Rizzo indicated that acquiring Jackson makes the Nationals no more likely to trade one of their starting pitchers. Of course, saying otherwise would hurt the Nationals’ leverage if they did discuss a trade with another team. But Rizzo called acquiring Jackson and making a trade “mutually exclusive.”

“We’re certainly always open to making a deal if it makes sense for us,” Rizzo said. “We did not acquire Edwin Jackson to trade another starting pitcher. In spring training or before spring training, if a deal comes up we can’t pass up that positively impacts our ballclub, we’d certainly be open-minded to it.”

The Nationals have a clear logjam in a rotation that ranks among the deepest and best in the National League — it now seems to line up as Stephen Strasburg, Gio Gonzalez, Jordan Zimmermann, Jackson and Chien-Ming Wang or Lannan.

While Jackson gives the Nationals an abundance of starters, the team is in position to acquire depth in its rotation. Pitching his first full season since Tommy John surgery, Strasburg will throw no more than about 160 innings. Last year, Wang returned from a shoulder operation that sidelined him for more than two years. Zimmermann has never pitched more than 200 innings.

In looking across the league, Rizzo found that six of the eight playoff teams included at least two starters who pitched 200 innings. The Nationals now have insurance should they suffer a spring training injury to their rotation. And if another team suffers one and becomes willing to overpay in a trade, the Nationals could step in.

“We had an innings shortage,” Rizzo said. “I like the competition aspect of this. There’s going to be a lot of good pitchers out there in spring training this year. The best 25 guys will go north.”

Rizzo began discussing a deal with Scott Boras, the agent Jackson shares with several high-profile Nationals, about 10 or 12 days ago. Once Rizzo realized Jackson would be open to a one-year deal, the idea of signing him became “much more palatable for us,” Rizzo said.

“The term and the value was too good to pass up,” Rizzo said. “We felt it improved our club immensely. There comes a point where his value was such that we were comfortable making the deal.”

Jackson spurned longer-term offers from teams in the American League to sign with the Nationals. The one-year deal gives him the chance to pitch in the less challenging National League before hitting the free agent market again next winter, he hopes, with a rebuilt value.

Jackson has some of the best pure stuff in baseball. Last season, his fastball averaged 94.5 mph, fifth fastest in the major leagues, and his slider zipped at 87.4 mph, sixth fastest. In 2010, he threw a 149-pitch no-hitter for the Arizona Diamondbacks.

But Jackson has also yet to produce a true breakout season while bouncing around with six teams since 2003, most recently the St. Louis Cardinals in the second half of last season. Last year, with the Chicago White Sox and the Cardinals, Jackson had a 3.79 ERA over 1992 / 3 innings, striking out 148 batters and walking 62. His best season came with the Detroit Tigers in 2009, when he made the all-star team and had a 3.62 ERA with 161 strikeouts in 214 innings.

The disconnect between his stuff and results may derive from his delivery. Some scouts believe Jackson shows the ball to hitters early in his release, making it easier for hitters to pick the ball up — and limiting the effect of his velocity. Rizzo believes Jackson still has “upside,” in part because of his release, which the Nationals perceive as a correctable flaw that surfaces only in his windup, not when he pitches from the stretch. The Nationals will work to make “tweaks” to his delivery, Rizzo said.

Over the past three seasons, the league has hit for a .283 batting average, .344 on-base percentage and .438 slugging percentage with no runners on base against Jackson, when he is pitching with a windup. The league has hit .246/.308/.385 with men on, when he’s pitching from the stretch.

“We’re going to make a few tweaks to his delivery,” Rizzo said. “He was a different pitcher out of the windup than out of the stretch. The numbers are really surprising.”

The question now will be, how do the Nationals make room for Jackson? Even after Lannan and Detwiler, the Nationals have Tom Gorzelanny, who began last season in their rotation and spent the majority of the year as a starter.

Detwiler, because of his youth and the fact that he is under team control for four seasons, would have far more trade value than Lannan. Detwiler is out of minor league options, which means the Nationals cannot send him to the minors. The Nationals cannot trade Wang until early summer because he signed this offseason as a free agent.

Lannan, whom the Nationals defeated in salary arbitration in a trial Wednesday, has an option remaining. But Lannan is an accomplished starter — he has a 4.03 ERA in 534 innings over the past three years, compared with 3.96 in 623 innings for Jackson — and Rizzo has no desire to send him down.

“We feel that he’s a major league-caliber starting pitcher,” Rizzo said. “He’s a major league starter, and he’s ready to help a contending team. That’s what we’re going to use him as.”