The Royals' ace tumbled to the ground at Progressive Field while covering first base on Michael Brantley's infield single.
Duffy has a 3.54 ERA over 68.2 innings in 11 starts this season, compiling a 4-4 record. In postgame comments, Duffy and manager Ned Yost did not say anything was physically wrong.
The southpaw was lit up by the Indians on Sunday, giving up six runs over just four innings.
"I really tried to bust over there and overran the bag", Duffy said. I'm very bummed out today. But I know the boys are going to pick me up. I'll be fresh and ready to go in six to eight (weeks).
The Royals purchased the contract of left-hander Eric Skoglund from Triple-A Omaha. He is 40-37 with a 3.70 ERA in 145 appearances (117 starts) and made nine relief outings during Kansas City's back-to-back World Series trips in 2014 and 2015.