Jeffress is on the Venezuelan Winter League roster of Bravos de Margarita, while Bueno will be in the Dominican Winter League with Tigres del Licey.
The hard-throwing Jeffress will be working to improve his command. In 13 games and 13 1/3 innings, he had a tidy 13 strikeouts and a messy 13 walks along with a 6.75 ERA, highest on the Kansas City staff.
Bueno, who quietly made a solid impression at age 31, had a 1.56 ERA in 18 games after a good showing as a Triple-A Omaha reliever. He also stranded all 14 runners that he inherited with the Royals.
Five players on MLB.com's Top 20 Royals Prospects list are also in the winter ball scene.
Pitcher J.C. Sulbaran (No. 8), shortstop Orlando Calixte (No. 14) and outfielder Brian Fletcher (No. 15) are with the Surprise Saguaros in the Arizona Fall League. Shortstop Christian Colon (No. 11) is with Leones de Ponce in Puerto Rico and outfielder Jorge Bonifacio (No. 16) is with Tigres del Licey in the Dominican.
The Royals currently list 21 players who are already playing in winter ball or have club approval to do so. Other players are likely to emerge later in the winter leagues. Catcher Salvador Perez and shortstop Alcides Escobar indicated they likely would play in Venezuela at some point.
Others currently on winter rosters:
Arizona -- Shortstop Alex McClure, second baseman Whit Merrifield, pitchers Edwin Carl, Jon Keck and Justin Marks, Surprise Saguaros.
Venezuela -- Third baseman Mario Lisson and pitcher Brandon Sisk, Navegantes del Magallanes; catcher Max Ramirez and pitcher Blaine Hardy, Bravos de Margarita.
Dominican -- Pitcher Michael Mariot, Tigres del Licey.
Puerto Rico -- Pitcher Ryan Dennick, Indios de Mayaguez.
Mexico -- Infielder-outfielder Anthony Seratelli, Venados de Mazatlan, and pitcher Federico Castaneda, Tomateros de Culiacan.
Australia -- Outfielder Carlos Testa, Melbourne Aces.