The injury isn't considered serious and the Royals expect Duffy to start next week against Detroit. He's 1-2 with a 3.63 ERA in three starts.
It's expected the Royals will announce Friday's starter following Thursday's game against the Indians.
Outfielder Lorenzo Cain will likely be sidelined for a month with a strained left hip flexor. Cain was on a minor league rehab assignment while recovering from a strained left groin.