Shields allowed two earned runs over six innings in the Royals' Game 3 win over the Los Angeles Angels in the ALDS on Sunday.
Shields' other postseason start came against the Oakland Athletics in the American League Wild-Card Game; he yielded four earned runs over five innings.
Over 227 innings this season, Shields has posted a 3.21 ERA with 180 strikeouts and 44 walks. The Royals' starters for subsequent ALCS games have not been revealed.
The 32-year-old Shields will have four days of rest before taking the mound in Game 1 on Friday at Camden Yards.