The Nationals needed a fifth starter because they didn't want right-hander Tim Redding to throw on short rest.

Cordero, who insisted that his shoulder is getting better, will throw a bullpen session on Saturday and most likely will be activated on April 11. Cordero has had shoulder problems dating back to Sunday when he was playing catch at Nationals Park.

"We thought it was the best for him because even when he feels good, [the arm] feels a little weak," manager Manny Acta said. "We really don't want to rush him back. He will be able to throw his side sessions and get him ready. That way we can bring in Johnny and give him his start."

Lannan was the Nationals' best pitcher during Spring Training. going 2-0 with a 2.18 ERA in five games. He was the Nationals' Minor League pitcher of the year in 2007.

In the meantime, with Cordero out, Jon Rauch will remain the closer.

This story was not subject to the approval of Major League Baseball or its clubs.