MILWAUKEE -- Nationals reliever Tyler Walker has been shut down because of an ailing shoulder. Walker, who is on the 15-day disabled list because of the injury, made four rehab appearances and was hit hard, allowing six runs in 4 1/3 innings.

"Walker is not feeling that good," manager Jim Riggleman said. "He has not responded as well as we would have liked. He threw a game, and he was OK. He threw another game, and the shoulder wasn't OK."

There is no timetable on when Walker will pitch again.

Injury to Martin extends Balester's stay

MILWAUKEE -- The Nationals recalled right-hander Collin Balester from Triple-A Syracuse on Saturday afternoon.

He was expected to replace right-hander Luis Atilano for a day and be sent back to Syracuse. But there was a change of plans. Balester will remain with the Nationals for the foreseeable future because right-hander J.D. Martin was placed on the 15-day disabled list with a bulging disk in his lower back after the team's 4-3 loss to the Brewers.

"I was a little disappointed to hear [about the disabled list], but I wasn't too surprised by it," Martin said. "I kind of expected that I would miss my next start."

Martin started for Washington on Saturday and allowed one run in 2 1/3 innings.

With two outs in the bottom of the second, Nationals manager Jim Riggleman, pitching coach Steve McCatty and head athletic trainer Lee Kuntz went to the mound to check on Martin. Catcher Ivan Rodriguez was the first to notice that something was wrong.

Martin said he was OK and remained in the game, but he would later leave after one out in the third, with runners on first and second.

Martin has had problems with his lower back for several years and has not needed surgery.

Martin's back problems flared up early this season when he was with Triple-A Syracuse. He never went on the seven-day DL, but missed two starts.

"I tweaked my lower back. I had some pain going down my leg, and that was about it," Martin said.

The team will recall left-hander Ross Detwiler to start Sunday's game against the Brewers in Atilano's spot.

As for Balester, he was brought back to the Major Leagues in case Martin had a poor outing against Milwaukee. Balester did, indeed, replace Martin and went two innings, allowing a solo homer to Jim Edmonds.

After six-plus years as a starter, Balester was converted into a reliever this spring. In 26 appearances -- five as a starter -- Balester is 2-3 with 7.27 ERA for Syracuse.

The Nationals wanted to save long reliever Miguel Batista for Sunday. Detwiler is on a pitch limit, because he is pitching on three days' rest.

But the bullpen is spent for Sunday, because the Nationals ended up using six pitchers in Saturday's game.

Riggleman went so far as to say that starter Livan Hernandez could pitch in relief. Hernandez said he could give the team at least two innings if needed.

The Nationals don't need a starter to replace Martin until Friday. Left-handers John Lannan and Matt Chico have a shot to be in the rotation. The team could also use a starter-by-committee approach starting with Batista, but that will depend on how much the relievers are used before that day.

Gonzalez gets opportunity as Dunn rests

MILWAUKEE -- Nationals manager Jim Riggleman decided to give first baseman Adam Dunn the day off Saturday and put shortstop-second baseman Alberto Gonzalez at first base.

Riggleman said giving Dunn a day off was planned for several days. The skipper said third baseman Ryan Zimmerman will get the day off Sunday, with Willie Harris getting a start.

"It's a combination of getting Alberto a game and giving Adam a day off," Riggleman said. "I prefer to do it on the road than at home, because I know our fans would come out every day to see Ryan and Adam. I have to take this opportunity to get Gonzo in the ballgame. Alberto is a ballplayer. I can put him anywhere."

It will be the first time in his professional career that Gonzalez will play first base. He doesn't own a first baseman's glove, so he will use Dunn's. He started practicing at the position about a month ago on the suggestion of Tim Foli, the senior assistant to general manager Mike Rizzo.

Besides first base, Gonzalez has been working out in the outfield. Gonzalez said he is willing to play those positions to get more playing time. Entering Saturday's action, Gonzalez is 23-for-78 (.295) with three RBIs.

"I feel good. I feel happy that I can play," Gonzalez said. "I have been practicing everywhere."

Peralta's success earns him responsibility

MILWAUKEE -- Entering Saturday's action, Nationals reliever Joel Peralta had allowed one run in 14 2/3 innings. Look for him to see more action in the late innings, manager Jim Riggleman said.

Since his contract was purchased in late June, Peralta has pitched in short and long relief.

"Joel has done a good job," Riggleman said. "He has really thrown a lot of strikes -- a nice repertoire of pitches -- and comes right at hitters. We are going to try to work him in some more situations where we have a lead and are in closer ballgames. He has had lot of good appearances. You get to the point where it's not a small sample of work. It's fairly a large number of appearances with a lot of success."