Buehrle had been Washington's top target since before Thanksgiving. The team would have liked him to lead a young pitching staff, which includes Stephen Strasburg and Jordan Zimmermann.
Buehrle has thrown at least 201 innings in each of the past 11 seasons, and he has been regarded as a good presence in the clubhouse. He is also not a stranger to the National League, having gone 24-6 with a 3.32 ERA during Interleague Play.
The Nationals talked to Buehrle's representatives late Tuesday night and on Wednesday.
"We are going to move on to our Plan B and see if we can help ourselves," Rizzo said. "We feel pretty good about how the Buehrle thing went. It was a good presentation and a good negotiation. He went where he felt most comfortable."
Rizzo believes the team already has good depth at starting pitching, which includes Strasburg, Zimmermann, John Lannan, Tommy Milone and Brad Peacock. However, Strasburg, currently the ace of the staff, will be on an innings limit in 2012 after recovering from Tommy John surgery.
The Nationals will talk to free-agent right-hander Roy Oswalt, who wants a three-year deal. But the two parties haven't had much dialogue recently. If Washington is able to sign Oswalt, it wouldn't have to give up a Draft pick because the Phillies didn't offer him arbitration.
Early in the free-agent process, the Nats were thinking along the lines of a one-year deal loaded with incentives, because Oswalt made a career-low 23 starts in 2011 due to back problems.
Oswalt has pitched over 200 innings in a season seven times. This past season, the 34-year-old went 9-10 with a 3.69 ERA. He missed over a month because of back issues. The Phils declined Oswalt's option, making him a free agent, and the Nationals believe he is healthy enough to make an impact in their rotation.
During his 11-year career, Oswalt is 159-93 with a 3.21 ERA. He has won 20 games in a season twice.
"He would be one of the options we look at," Rizzo said.
The Nationals have also expressed interest in Athletics left-hander Gio Gonzalez, who is arbitration eligible. This past season, Gonzalez was 16-12 with a 3.12 ERA and 197 strikeouts in 202 innings.
"We have spoken to Oakland. He fits what we are looking for," Rizzo said. "He is young, controllable, talented starting pitcher that has a proven track record. With that said, those guys come at a cost. We have to balance what the cost is and what the game is to a club long term."