NEW YORK -- Outfielder Corey Brown underwent surgery on Wednesday because of a staph infection in his right knee, and he is out for the season.

Manager Davey Johnson learned about Brown's problem on Sunday, when the Nationals were preparing to depart for New York. Team doctors then suggested to Johnson that Brown would be better off if he didn't travel.

Brown went hitless in three at-bats after his promotion to the big leagues last Tuesday.

Brown was part of the deal that sent Josh Willingham to the Athletics in December.

Desmond finds that Johnson's lessons pay off

NEW YORK -- With the help of manager Davey Johnson, shortstop Ian Desmond has found his stroke at the plate. Entering Wednesday's action against the Mets, Desmond was hitting .279 with a .330 on-base percentage since the All-Star break.

Among other things, Johnson told Desmond to stop taking every pitch to right field.

"He was trying to shoot the ball [to right field] too much," Johnson said. "He has a lot of power. I want him thinking more about hitting the ball where it's pitched -- get doubles, get out in front a little bit and drive it. He has been doing that.

"Ian Desmond is his own best coach. Sometimes he can get a little off track. At times he has tried to do a little too much. When he stays within himself, he is outstanding."

Johnson went so far as to say that based on Desmond's success in the second half, he could see Desmond as his leadoff hitter next year.

"I like him leading off," Johnson said. "I know he feels real comfortable about it. I remember when I had that conversation with him. His on-base percentage was terrible, but he was trying to do too much. He was expanding the zone. But I also wanted to know what he thought. He said, 'I think I could be a great leadoff hitter.' So I said, 'If you think that, show me.'

"I put him in the leadoff spot, and he showed me that he can be more patient and be the kind of hitter [I expect him to be]. ... In the last month, he has shown the real Ian Desmond."

Mattheus throws 'pen session without problems

NEW YORK -- Right-hander Ryan Mattheus, on the disabled list because of a sore right shoulder, had his second bullpen session on Tuesday without any problems. He threw 25 pitches, and included split-fingered fastballs and sliders.

"[The bullpen session] was great. Better than the first one," Mattheus said. "The ball is coming out real good. I had no issue with the arm, which is the biggest thing. ... I let it go a little bit more and it felt game like. My stuff was really good."

Mattheus will have another bullpen session on Wednesday. If everything goes well, Mattheus hopes to have a simulated game when the team returns to Nationals Park this weekend.

Mattheus is expected to be activated by Sept. 20, when the Nationals play a doubleheader against the Phillies in Philadelphia.

Mattheus has a become a reliable reliever for manager Davey Johnson. In 32 games, Mattheus has a 2.40 ERA.