Pena replaced outfielder Coco Crisp, who went on the bereavement list because of his great-grandmother's death Monday. To make room for Soria, right-hander Sidney Ponson was placed on the 15-day disabled list with a right elbow strain.
Manager Trey Hillman said Soria would immediately resume the closer role.
"Not only physically, but I think mentally it should stabilize us a little bit better in the bullpen," Hillman said. "It puts guys more in a comfort zone, and the confidence level will go up just having Jack back."
Soria returned with seven saves, one win and a 2.08 ERA in eight games. After he worked in two intrasquad games in Arizona with no ill effect, the Royals decided he was ready to pitch.
The closer had not pitched because of a sore right shoulder since May 7, the day he had a rocky inning against Seattle. He gave up just one run, but he faced seven batters and made 29 pitches.
"That inning, I didn't feel anything at all. The next day was when I was feeling the pain," Soria said. "In Arizona, I pitched a 25-pitch inning and the next day I felt good, so that means I'm ready to go."
Pena had missed 26 games since going on the disabled list because of left hand surgery on May 2. He was 0-for-11 in 10 games, but he had a chance for his first hit Tuesday night -- as he was in the starting lineup against Tampa Bay.
Hillman wasn't certain how frequently Pena would play because he wants to see how his left hand reacts to game action and also how he holds up to the rigors of nine-inning games, which he hasn't experienced regularly this year.
"If we're going to get things turned around win-wise, we've got to be able to pitch it and catch it as much as we can, especially on the artificial turf here and in Toronto," Hillman said.
"Hopefully, it plays out that way and he gets a lot of ground balls and he can do some things defensively and help us at the bottom of the order as well. He plays with good energy, and hopefully that rubs off on some of his teammates."
Also available to play shortstop are Luis Hernandez and Willie Bloomquist, but Pena gives the Royals better lateral range, Hillman said.
Mike Aviles, who started the season as the regular shortstop, could be out for an extended period of time. He went to Los Angeles on Tuesday to get a second opinion on his ailing right forearm from Dr. Lewis Yocum, the noted orthopedic surgeon.
"Mike hasn't progressed," Hillman said. "It could be a while."
Ponson last pitched against Chicago last Friday, when he faced six batters but got just one out, giving up four runs on four hits and a walk. He is 1-5 with a 7.27 ERA in 11 games, the first six of them starts. His 15 days on the DL began retroactively on Saturday.
Ponson said he first experienced pain and inflammation in his elbow in that game.
"He was still throwing it hard. But throwing it hard and throwing it where you want are two different things," Hillman said.
"Sidney has taken the ball every time we've asked him to take the ball, but physically, if we climbed inside him, I don't think he's been comfortable with his elbow since going from the rotation to the 'pen. But he's not a complainer."
Hillman reported that catcher John Buck, on the disabled list with a herniated disc in his lower back, was dismissed from the hospital and is getting back to his everyday living routine.
Catcher Brayan Pena, summoned to take Buck's place, did not arrive in Kansas City until well after Sunday's game. That meant that Miguel Olivo was the Royals' only catcher for the game against Chicago but, fortunately, he didn't need a backup. Pena joined the club for the flight to Tampa Bay.
The Royals will have to make another roster move when Crisp returns from his bereavement leave, which is expected to be before Thursday's game at Tampa Bay. The leave is for a minimum of three days, and because Crisp went on the list Monday, that time will be up by Thursday.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.