VIERA, Fla. -- The Nationals have decided that left-hander John Lannan will be their Opening Day starter against the Marlins on April 6 at Dolphin Stadium. An official announcement will be made by manager Manny Acta on Wednesday.

Left-hander Scott Olsen will pitch on the night of April 7, while righty Daniel Cabrera will start the next afternoon.

Lannan, who clearly is the Nationals' best pitcher, is coming off a rookie season in which he was 9-15 with a 3.91 ERA. This spring, Lannan hasn't given up an earned run in six innings.

"What impresses me about John is his command of all his pitches, the poise on the mound beyond his years and his competitiveness on the mound," assistant general manager Mike Rizzo said. "He is a soft-spoken guy off the field, but, on the mound, he is a tiger and he goes after hitters. That separates him from a lot of pitchers in the National League."

An 11th-round pick in the 2005 First-Year Player Draft, Lannan's rise to the big leagues was rapid. Two years after being drafted, Lannan went from Class A Potomac to the Majors in one season and had a respectable 4.15 ERA in six big league games. By early last season, Lannan was the team's No. 1 starter.

Lannan, 24, was not available for comment.