Iwamura felt the hamstring begin to tighten as he ran to first in the seventh inning of Saturday's 4-3 win. He was removed from the game immediately after and is expected to miss 2-4 games while he lets his hamstring heal.
"The good thing," Iwamura said through a translator, "was I was able to stop before I made it worse."
The Pirates don't anticipate having to place Iwamura on the disabled list, and manager John Russell even noted that the infielder would be available to pinch-hit Sunday, if necessary.
The injury comes with Iwamura mired in a deep offensive funk. He is batting just .159 on the season, hitless in his last 26 at-bats, and just 2-for-43 this month.
"With the numbers I have, I don't think I have time to sit down and not play," Iwamura said. "The timing was bad for me to have this kind of injury. It's frustrating."
Despite the numbers, both Iwamura and Russell expressed some encouragement with Iwamura's performance on Saturday. He showed some plate patience and drew two walks as a result. Iwamura also squared up a ball that was caught up against the right-field fence.
"It's unfortunate his hamstring is tight, because I think he was starting to do some things that will get him headed in the right direction," Russell said. "He's doing some better things. Hopefully he's not out too long."
Russell will use Bobby Crosby and Delwyn Young to fill in at second base during Iwamura's absence.