Wood, a 6-5 right-hander, was called up from Triple-A Omaha on Wednesday and Rupe was designated for assignment.
For Omaha, Wood worked 12 games in relief and posted five saves with a 2-1 record and 2.16 ERA. In 16 2/3 innings, he had given up 12 hits, seven walks and four earned runs with 12 strikeouts. He pitched one scoreless inning on Tuesday night in a 10-2 victory at Reno.
"We'll use Blake as needed. He's been throwing the ball well," manager Trey Hillman said. "He's been used in back-to-back games, multiple innings; we look at him as a two-inning guy max. We'll use him the same way we've used other guys to try to get to Jack [closer Joakim Soria]."
Hillman wasted no time, giving Wood his Major League debut on Wednesday night in the Royals' 4-0 loss to Cleveland. Wood worked a scoreless seventh inning, giving up a single to Asdrubal Cabrera but then ending the inning on a double play.
Rupe was 1-1 with a 5.59 ERA in 11 games for the Royals. He was called up from Omaha on April 16 when Roman Colon was designated for assignment.
The former Texas Rangers right-hander bottomed out in Tuesday night's game, allowing all four batters he faced to reach base. Rupe gave up two singles and two walks to force in a run in the Royals' 8-2 loss to the Indians. In his previous outing last Thursday night, he entered the game at Texas and promptly surrendered a two-run homer to Vladimir Guerrero.
"He has a previous outright, so he can elect free agency if he chooses," Hillman said. "I hope he doesn't, because I think the stuff will still play out provided he throws strikes."
Right-handed batters were hitting .391 (9-for-23) against Rupe.
"With his stuff, he should be more effective against right-handed hitters," Hillman said.
Wood, 24, has been one of the Royals' top pitching prospects since he was their third-round selection in the 2006 First-Year Player Draft. He was a starter until late last season when, with Double-A Northwest Arkansas, he was recovering from a strained right forearm and worked out of the bullpen. He liked it, and it worked so well the Royals decided to make the move permanent.
Showing a heavy fastball and an effective changeup, Wood had an impressive Spring Training. He appeared in seven games and had a 4.70 ERA but held batters to a .167 average. For Omaha this year, he's been more effective against left-handed batters (.194) than against righties (.222).
Wood is from the Atlanta area and played three seasons at Georgia Tech and reached the College World Series in 2006 as a junior.
"I don't have time to baby him," Hillman said. "We need to see what this guy's got, we've got to get to Jack with the lead."
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.