Guidry pitched the AL East division tie-breaker game against the Boston Red Sox and was starting to have arm trouble.
Also, second baseman Willie Randolph would miss the entire postseason with a hamstring injury and be replaced by a platoon of Fred Stanley and Brian Doyle.
Meanwhile, the Yankee bats knocked Dennis Leonard and Steve Mingori around for 13 hits and four runs, Doyle chipping in an RBI single.
Royals' starter Larry Gura pitched six shutout innings and won with relief help from Marty Pattin and Al Hrabosky.
Yankee starter Catfish Hunter pitched a fine game, going six innings, except for one thing: three consecutive home runs by George Brett.
The Royals missed an opportunity in the top of the sixth when Pete LaCock led the inning off with a triple, but Hunter retired the next three batters without the run scoring.
After a one-out single by Roy White, Royals manager Whitey Herzog replaced his starter, left-hander Paul Splittorff, with right-hander Doug Bird to face Thurman Munson.
Guidry left in the ninth after giving up a leadoff double to Amos Otis and Goose Gossage set down the next three Royal batters to close out the series and win their third straight AL pennant.