Showalter said the Orioles were waiting a few days before making their decision on O'Day because they weren't sure the injury warranted a stint on the DL.
Ultimately, they put him on the DL to ensure he wouldn't take the mound prematurely and risk making the injury worse. Showalter doesn't expect O'Day to be out long.
"We can wait day to day to day, and then he goes out and pitches and it flares back up," Showalter said. "We'll take this seven to eight days and put it behind him."
O'Day's injury further thins an already limited Orioles bullpen, which has been without closer Zach Britton since early May. O'Day is 1-1 with a 3.86 ERA in 24 games this season.
The Orioles recalled rookie right-hander Stefan Crichton from Triple-A Norfolk. He allowed six runs and 16 hits in 8 1/3 innings for the Orioles earlier this season before being sent down. The Orioles' No. 28 prospect fared better in his 12 games in Triple-A, posting a 1.37 ERA and a 9.33 strikeout-to-walk rate in 26 1/3 innings.
Machado still sore; Castillo aims for Saturday Manny Machado, who is out of the lineup for Friday night's series opener against the Yankees, is still sore from when Andrew McCutchen slid into and spiked his left wrist earlier this week. His status is day to day.
Showalter said he wants to wait a little while to see if Machado's wrist heals before deciding whether to put him on the 10-day DL.
"We'll reach that point with Manny if we don't feel like he can play every day," Showalter said. "I hope it's going to be OK tomorrow."
Catcher Welington Castillo played in a rehabilitation game Friday night in Trenton, N.J., with Double-A Bowie, and if all goes well, he will be activated from the DL in time for Saturday's game against the Yankees.
Matthew Martell is a reporter for MLB.com based in New York.