Kevin Gausman, Bud Norris and Miguel Gonzalez will start the three games against the Cubs at Wrigley Field, and Chris Tillman will pitch Monday night against the Rays at Camden Yards.
Jimenez has made one relief appearance, and it came in his major league debut on Sept. 26, 2006 against the Dodgers. He worked the eighth inning and allowed two hits.
Since then, Jimenez has made 231 starts in nine seasons.
Jimenez, who signed a four-year, $50 million contract with the Orioles in spring training, is 4-9 with a 4.83 ERA in 20 starts. He continues to lead the American League with 66 walks.
Jimenez is scheduled to throw a side session on Wednesday. Earlier today, he told reporters that he expected to stay in the rotation.
"I guess my next start is after (Wei-Yin) Chen, right?" he said. "I guess that's my next start. That's what I'm getting ready for."
Chen is starting Wednesday night in the series finale against the White Sox, allowing him to work on regular rest. Gausman and Norris also will pitch on their regular turn. Tillman will have an extra day.
Showalter wants Tillman to make three starts on the next homestand.
The Orioles must make a roster move on Sunday to accommodate Gonzalez, who's carrying a 3.80 ERA in 20 games (19 starts).
Jimenez, who allowed six runs in 4 1/3 innings in his last start in Cleveland, will squeeze into the bullpen for an undetermined period of time. He figured to settle into a relief role once rosters expanded in September.
"After today, he's probably available in the bullpen," Showalter said. "We'll see where it takes us as we go forward, where the need might be. If somebody might need an extra day or something. It depends who we send out to make room for Gonzo.
"I'm going to wait and see when we get there what our needs are. After the off-day (Thursday), I think we play quite a few days in a row, so it might be a moving target. That target is trying to get to Sept. 1. We've got less than two weeks."
Showalter made certain that he broke the news to Jimenez before announcing it to the media.
"I just got through talking to him. Dave (Wallace) and I and Dom (Chiti) talked to him," Showalter said, adding that it was a difficult conversation.
"You've got your reasons. He doesn't have to agree with them. He's been a great teammate and a good professional. He's got a good heart. He wants to do well. And it's not like he's pitched poorly all year. It's just that, with 39 games left, we'll see how Gonzo does. We don't have any inning limitations now on anybody.
"There will be a need that Ubaldo can serve for this team. It could be as a starter and it could be as a reliever. Right now, the next five games, or six if you count Tillman, we know where we're going."
Jimenez must adjust to pitching out of the bullpen and getting ready quickly. It's a foreign routine for him that could be challenging.
"I don't think anybody can sit here and answer that," Showalter said. "I know he'll embrace it as the need of the club that it is. A lot depends on whether we go with 13 pitchers or 12 and how that all works out. He's healthy, feels good. When we looked at him, he was one of the few guys who seemed like his health, knock on wood, wasn't going to be a factor.
"Hopefully, we can get him back a little bit to pitching like he has at times this year. It's going to be tough. He's not going to sit there and take a lot of work days. That's a very valuable job that he's going to have to do now and then."