Duffy was put on the 15-day disabled list the same day an MRI revealed the seriousness of the injury, and the Royals said he is expected to get a second opinion from Dr. Lewis Yocum in Los Angeles.
Duffy is the second Royals starter to go on the disabled list in less than a week. Right-hander Jonathan Sanchez was put on the 15-day DL last week with left biceps tendinitis.
The 23-year-old Duffy (2-2, 3.90 ERA) faced only three batters in Sunday's 9-1 win over the Chicago White Sox. After catcher Humberto Quintero saw Duffy shake his arm after a throw, manager Ned Yost and a trainer visited the mound. Duffy was pulled when he said his elbow was the problem.
Duffy also experienced tightness in his elbow last month, forcing him to skip a turn in the rotation.
"The thing about these Tommy John surgeries nowadays, they're so efficient at doing them. Danny is going to come back even better, even stronger," Yost said before the opener of a two-game series in Texas. "It's just missing out on Danny's development for a year and Danny's production for a year which is what hurts. He was really starting to come into his own. ... He was throwing 95-97 mph, he had a great start and he's starting to learn how to command it."
"We want to see Vinny. We know what Nate can do," Yost said. "We're trying to sort some things out. We're looking for two starters. ... We got to give Vin a shot."
Duffy was drafted by the Royals in the third round in the 2007 draft and made his major league debut last season, going 4-8 with a 5.64 ERA in 20 starts. In six starts this season he had a team-leading 28 strikeouts over 27 2/3 innings.
Sanchez went on the DL a day after he lasted only three innings in a loss at Boston on May 7. Sanchez (1-2, 6.75) was acquired in an offseason trade with San Francisco to bolster the starting rotation.