Guillen and Gibbons each faced 15-day suspensions at the start of the season for violations of the Commissioner Bud Selig's drug prevention program.
However, a report on Thursday indicated that the two players could have their suspensions revoked under an agreement between MLB and the Players Association. The report speculated the two sides were close to an agreement.
Guillen, the Royals' newly-acquired right fielder, is likely to bat fourth in the team's Opening Day lineup against Tigers right-hander Justin Verlander. Manager Trey Hillman had talked about starting utility player Esteban German in right field if Guillen were still suspended.
"We're certainly glad to have him in the lineup," general manager Dayton Moore said. "We signed him to play. We want him in there every day. It makes our team better on March 31."
Guillen got a late start playing Cactus League games and then was knocked out a week by the flu. But Hillman doesn't think the down time will have an adverse effect.
"I believe he's going to be just fine," Hillman said. "We'll play him today and tomorrow and see how he does."
The Royals have a two-game series at Milwaukee on Friday night and Saturday afternoon before moving to Detroit for Monday's American League opener.
Guillen played his 10th spring game on Friday night and had an RBI single in three at-bats against the Brewers. Overall, he's just 4-for-25 (.160) with one home run on the spring.
Guillen and Gibbons were suspended on Dec. 6. A week later, the Mitchell Report recommended that players cited in the document not be penalized. Selig indicated he expected that all of former Sen. George Mitchell's recommendations would be adopted.
On Thursday, MLB informed the Royals that catcher Miguel Olivo's five-day suspension for fighting was reduced to four days. He'll be eligible to play on April 5 at Minnesota.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.