Royals first baseman Eric Hosmer added to his AL Rookie of the Year case on Thursday when he was selected the AL Rookie of the Month for September.

Hosmer batted .349 with five homers and seven doubles while leading all rookies with 41 hits in 25 games. He highlighted his September with a five-for-five performance in a victory over Detroit on Sept. 20.

Hosmer, 21, finished the season batting .293 with 19 homers, 27 doubles and 78 RBIs in 128 games after making his major-league debut against Oakland on May 6 at Kauffman Stadium.

He also took home AL Rookie of the Month honors for July.

On Thursday, Royals GM Dayton Moore said Hosmer's progress had surpassed all expectations entering spring training.

Club officials previously hoped last offseason that Hosmer would be ready for full-time duty in 2012, Moore said. But those estimates quickly changed when Hosmer showed up at spring training.

"It was very clear to us," Moore said, "that he was probably one of the top two players in the entire Cactus league, talentwise."

Two other Royals rookies — third baseman Mike Moustakas and catcher Salvador Perez — also received votes for the award.

Focus is on the rotation

Moore began the offseason Thursday by reaffirming the club's desire to explore every option in attempting to upgrade its starting rotation.

The Royals' starters finished the season with a combined 4.82 ERA — second worst in the American League. That number mostly nullified a young and improved offense, which finished the year sixth in the league with 730 runs scored.