"He's not able to go back-to-back days, and that's critical, especially with the fact we're going to go 20 days in a row starting Friday," Reds manager Dusty Baker said of Marshall. "It taxes the rest of the bullpen."
Marshall, 30, was unavailable much of last week because of fatigue in his shoulder. The club hoped rest would help, and it did not use Marshall until Sunday vs. the Nationals, when he pitched a perfect seventh inning with one strikeout.
"Monday, it didn't feel great," Marshall said. "I would've liked to have bounced back, but it was my first appearance in a while. We thought it was the best idea, because the bullpen has been pitching and working very hard, to have somebody fresh that's able to pitch consecutive days. I was close to being able to do that.
"I felt like it was in the best interest of myself and the rest of the guys to take this DL stint, and really continue to strengthen my shoulder and get it back to 100 percent and pitch the rest of the year without any problems."
The transaction is retroactive to Monday. Because Marshall pitched Sunday, the club could not backdate the move to before the beginning of the season.
An MRI was performed on Marshall's shoulder, but no structural damage was found.
"I know myself pretty well," Marshall said. "I've been ready to pitch almost every game for three or four years. My shoulder just didn't feel like its normal self."
Marshall has pitched 366 big league games over his career, including a team-leading 73 appearances last season. His only previous DL stint was in 2006 with the Cubs, when he suffered a strained oblique.
Ondrusek was one of the final cuts in Spring Training, during which he struggled. The club sent him to Double-A to work with pitching coach Tom Brown and try to get his stuff together again. Ondrusek worked 2 1/3 scoreless innings in three appearances for Pensacola.
"They said he's been throwing pretty good," Baker said.
Mark Sheldon is a reporter for MLB.com. Read his blog, Mark My Word, and follow him on Twitter @m_sheldon. This story was not subject to the approval of Major League Baseball or its clubs.