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.