Yost and pitching coach Dave Eiland met with Duffy hours before the Royals wrapped up their four-game set with the Indians.
"He hasn't had a horrible year," Yost said of Duffy. "But we're getting to the end of the year, and we only need four starters for the playoffs. If this was in the middle of the season, we probably wouldn't be talking about this.
"We've got 17 games left. You need four starters, you don't need five.