Rafael Soriano just threw a curveball and Frank Wren didn’t see it coming.

In a move that the Atlanta general manager hadn’t planned for, Soriano has accepted the arbitration offer from the Braves and will return to the club for the 2010 season.

Soriano, 29, racked up 27 saves in 31 chances last season. He also posted a 2.97 ERA and a 102/27 K/BB ratio in a career-high 75 2/3 innings.

Soriano made $6.5 million in 2009 and figures to make close to $8 million via arbitration.

On the bright side, with Soriano returning, the Braves figure to have one of the most dominant bullpens in all of baseball next season.

On the not so bright side, with Soriano returning, the Braves figure to have one of the most expensive bullpens in all of baseball next season.

Wren offered arbitration to both of his free agent relievers, Soriano and lefty Mike Gonzalez with the intent of reaping first-round draft picks when the two signed elsewhere.

Needless to say, Soriano’s decision to accept the offer throws a wrench into those plans and the rest of the Wren’s offseason to-do list.

That list included rebuilding the backend of the bullpen after the loses of Gonzalez and Soriano, something Wren took care of last week when he dished out big bucks to sign closer Billy Wagner and setup man Takashi Saito.

After investing more than $10 million in his bullpen, Wren was ready to move on to the second item on his list, adding some offense via free agency and trades.

Soriano’s expected 2010 salary will undoubtedly impact Wren’s already limited funds to acquire a big bat and will definitely force the club down the path of trading from an enviable surplus of starting pitchers.

It is believed that Wren has already dipped his toes into the trading pool, but hasn’t found much interest in either Derek Lowe or Javier Vazquez.

The Braves figure to struggle to find a taker for Lowe and the three years and $45 million that remain on his contract.

Vazquez is only under contract through 2010 at $11.5 million, but was easily the club’s most reliable starter in 2009 and the Braves would no doubt be hesitant to lose his presence at the front of the rotation.

Additionally, Vazquez has a limited no-trade clause that prevents the club from moving him to any team in the National League West or American League West.

Neither pitcher figures to have much value until after John Lackey is signed to a new deal. After Lackey, the free agent pool of starters is very shallow and many teams could find Lowe and Vazquez to be more attractive alternatives.

The Braves may attempt to trade Soriano, but he’ll be in control of the situation more than the club will.

As is the case with all signed free agents, Soriano now possesses a full no-trade clause until June 15, so the Braves cannot move him to another club without his permission.

Clubs such as the Tampa Bay Rays, Houston Astros, and Baltimore Orioles figure to be more interested in acquiring Soriano now that he won’t cost a first-round draft pick.

It is also possible that the club could release him in Spring Training—as arbitration players don’t receive guaranteed contracts—if his performance gives them any reason to do so.

It is conceivable that Atlanta could choose to ride out the storm with Soriano, Saito, and Wagner anchoring the bullpen. All three are proven injury-risks and all three have proven they can thrive as setup men or closers.

The Braves would be heavily invested in the rotation and bullpen, but could bid on some bargain free agent options and/or call up top prospect Jason Heyward to fill the hole in the outfield and, hopefully, in the lineup as well.

Whatever Wren does (or doesn’t do) going forward will undoubtedly be affected by Soriano’s decision to accept arbitration.

If there is one thing Wren is learning about pitchers this offseason, it’s that sometimes they throw curveballs.