Greinke will replace Odalis Perez, who might be headed for the disabled list because of a sore left knee. Perez was in Kansas City on Tuesday for an MRI.
That wasn't the only medical news.
Manager Buddy Bell scratched right fielder Mark Teahen from Tuesday night's lineup against Chicago and speculated that he might be a DL candidate, too.
Teahen has a hyperextended left elbow, suffered last Saturday night diving for a ball at Oakland. This was the third straight game that he's missed.
He was in the lineup until he took some swings in the batting cage late Tuesday afternoon.
Perez has an 8-11 record after back-to-back victories over Toronto and Oakland. He's made 26 starts and has a 5.57 ERA.
Greinke began the season in the rotation and made seven starts. But he was 1-4 with a 5.71 ERA and was moved to the bullpen where he prospered.
"I've never seen him more confident," Bell said. "[Starting] is what he wants to do."
Bell said that Greinke will start for the remainder of the season as part of what is envisioned as a six-man rotation during September.
"I think you're more valuable to a team as a starter," Greinke said. "So, if you're good, then it's better to be a starter. It's harder to be good as a starter, in my opinion."
As a reliever used mostly in middle relief and setup situations, Greinke was in 38 games and had a 3.54 ERA, a 4-1 record and one save. In 53 1/3 innings, he struck out 55 and walked just 15. Opponents batted .226 against him.
"I always thought it'd be good for me to do the bullpen for a year or a half year because my arm has never gotten used to throwing as hard as it could. I always thought that throwing in the bullpen, I could get my arm speed up and, hopefully, my stuff improves," Greinke said.
"I assume I can throw it [hard] consistently for one inning or two, sometimes even three. I think I can, but I don't know for sure if I can do that for full game."
Bell said he'd "piggyback" Greinke with another pitcher because his pitch count will be limited after four months in the bullpen.
"It'll be more like using a bunch of relievers instead of a starter and relievers," Greinke said. "But September's coming soon and they'll have lots of support coming in."
When Bell does kick in his six-man rotation, it's likely to include Jorge De La Rosa, currently on the DL because of a strained elbow. De La Rosa is scheduled for an injury rehabilitation start for Double-A Wichita on Wednesday night and another before returning on Sept. 1.
Brandon Duckworth also is working his way back. On Tuesday, he pitched six innings for Triple-A Omaha and gave up one run, Valentino Pascucci's 33rd homer, in a 5-1 loss to Albuquerque. Duckworth threw 71 pitches as he comes back from a rib cage injury.
Although there will be six starters, Bell will try to have Gil Meche continue to work every fifth day.
Sweeney plan: The Royals finally clued in team captain Mike Sweeney on plans for his rehab assignment with Omaha.
"I'll head out Thursday and drive to Omaha and play a few games up there, then accompany them to Nashville. Then I'll be ready to go," Sweeney said.
He said the plan is for him to be activated on Sept. 1 when the 25-man rosters are expanded.
"It's two weeks longer than I planned on but the team makes the decisions. I'm just following orders," Sweeney said.
"I'm feeling good and my hope is to get going and play the whole month and help these guys get ready for next year -- with me or without me."
Sweeney's contract expires this year and his Kansas City future remains unknown.
You're the top: Joey Gathright, on a hitting surge, was the leadoff batter in the Royals lineup for the first time this season Tuesday night against the Chicago White Sox.
Bell also dropped David DeJesus, his leadoff batter most of the season, into the No. 3 spot, with Mark Grudzielanek hitting No. 2. For DeJesus, batting third was a season first, but he'd done it eight times in 2006.
When Teahen was scratched, Emil Brown replaced him in right field and in the No. 6 spot in the order.
Royals rundown: Mike Stodolka had three hits, three runs and two RBIs in Wichita's 7-5 win over Frisco on Monday. ... Derrick Robinson stole his 33rd base for Class A Burlington (Iowa), in a 1-0 loss at Cedar Rapids. ... Clinton Robinson's 13th homer, a three-run shot in the ninth, gave rookie Idaho Falls a 9-7 victory over Billings. Anthony Seratelli hit his eighth homer and is batting .335. ... Jake Lane's two-run single pulled rookie Burlington (N.C.) ahead in a 9-7 win at Greeneville. Patrick Norris had three hits and swiped his league-leading 27th base.
Coming up: The Royals wrap up their trip against the White Sox at 1:05 p.m. CT on Wednesday at U.S. Cellular Field. Right-hander Brian Bannister will start for the Royals; right-hander Jose Contreras gets the nod for the Sox.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.