The Royals made the announcement on Tuesday morning, continuing a pitching shakeup that began on Monday with Kyle Davies' injury and Vin Mazzaro's drubbing in a 19-1 loss to the Indians.
Mazzaro was optioned to Triple-A Omaha immediately after giving up 14 runs in the game and Teaford was brought up from the same club.
No announcement has been made on Davies, although he might go on the disabled list with a shoulder injury that caused him to leave Monday night's game in the first inning. If so, reliever Robinson Tejeda could be recalled from his injury rehabilitation assignment.
Duffy will have to be added to the Royals' 40-man roster as well as the 25-man active roster and the corresponding moves will be made when he is activated.
For Omaha, Duffy has a 3-1 record and a 3.00 ERA in seven starts. He has an impressive 43 strikeouts and just 10 walks in 36 innings while giving up 30 hits.
He's one of the Royals' brightest pitching prospects and was selected over fellow left-hander Mike Montgomery, who has a 2-1 record and a 4.25 ERA in eight games for Omaha.
"He's throwing the ball better, commanding the fastball, keeping it down," Royals manager Ned Yost said. "He did a nice job."
When Spring Training ended, it seemed that Montgomery was ahead of Duffy in the derby to reach the Majors.
"I think that was everybody's impression and Duffy overtook him," Yost said.
It's a big turnaround for Duffy, who left the Minor League Spring Training camp in 2010 to take care of personal matters and to decide on his future. But he ended his leave last June 2 and resumed pitching on June 28 for the Surprise, Ariz., Rookie team.
During last summer he moved up the ladder through the Idaho Falls, Wilmington and Northwest Arkansas clubs. He helped the Double-A Northwest Arkansas Naturals win the Texas League championship, going 1-0 with a 1.69 ERA in two playoff games after posting a 5-2 record and 2.95 ERA in seven regular-season starts.
After that, Duffy joined five other Royals farmhands on Team USA in the Pan American Games Qualifying Tournament and also pitched in the Arizona Fall League.
"Our scouts and [senior pitching advisor] Bill Fischer felt that he's as ready as he's going to be and just needs to take the next step," general manager Dayton Moore said.
"The thing that he has in his favor is he has 'out' pitches. He has a great changeup, a hard overhand curveball, a fastball that he throws up to 96 miles an hour. So it gives him a chance to compete and gives him an edge."
From Lompoc, Calif., Duffy was a third-round pick by the Royals in the 2007 First-Year Player Draft. He had a 9-3 record in a career-high 24 starts for Class A Wilmington in 2009 and, in five Minor League seasons, has a combined record of 27-14 and a 2.59 ERA in 73 games, all but two of them starts. In a total 344 innings, he has 402 strikeouts.
Duffy's insertion into the pitching rotation means a change in those plans for the third time in three days.
Sean O'Sullivan started Tuesday night's game against the Indians. Duffy will start Wednesday and Luke Hochevar on Thursday against the Rangers with Jeff Francis facing the St. Louis Cardinals on Friday.
Adcock is scheduled to make his first start on Saturday against the Cardinals in Davies' place.
Duffy, in effect, is taking the spot vacated by left-hander Bruce Chen, who is on the disabled list with a back strain and was replaced for one start by Mazzaro.
Chen said he's about ready to resume playing catch in the next day or two.
Although it's not certain how long Chen might be out, he joked that he wants to be back by the time the Royals begin Interleague play at National League cities so he can bat.
"I've got a hitting streak dating back to 2006. People can't get me out," Chen said.
Well, true enough, since then he is 3-for-3 in Interleague hitting. Even so, that segment of the Royals' schedule doesn't begin for a month and he'd certainly like to be back sooner than that.