Dr. Lewis Yocum, a noted specialist, will perform what is medically known as ulnar collateral ligament reconstruction on Friday in Los Angeles.
Lamb left a start for Double-A Northwest Arkansas in the first inning on May 19 at Tulsa and hasn't pitched since. He had a 1-2 record and 3.09 ERA in eight starts for the Naturals this year.
Just 20, he's one of the young rising stars in the Royals' farm system and is seen as an impact starter of the future. Lamb was ranked as the organization's No. 5 prospect by MLB.com, No. 27 overall in the Minor Leagues and No. 6 among left-handed pitchers.
Last season, Lamb won the Paul Splittorff Award as the system's top pitcher after a 10-7 record and 2.38 ERA for three different teams. His most impressive stat: 159 strikeouts in 147 2/3 innings.
Full rehabilitation from Tommy John surgery generally takes about a year.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.