[1] His fastball, tight slider and nasty forkball contributed to impressive pitching statistics, distinguishing himself as one of the American League's top right-handed starting pitchers throughout much of the 1990s.

Highlights of the season included a career-high nine-game winning streak and the American League Pitcher of the Month award for July, in which he went 4–0 with a 1.55 ERA.

Though he recovered from that injury, in March 1998 he needed surgery for an unrelated long-term progressing shoulder ailment, the repair of a torn labrum,[2] causing him to miss nearly the entire 1998 season.

In 1999, after losing faith in the direction of the Royals organization and eager to play with a contender, Appier requested to be traded.

In the middle of the 1999 season, Appier was dealt to the Oakland Athletics for three pitchers, Jeff D'Amico, Brad Rigby and Blake Stein.

A free agent after the season, Appier signed a four-year, $42 million contract with the New York Mets, with which he tied for the team lead in wins, going 11–10 with a 3.57 ERA and went undefeated in his final 12 appearances, going 6-0 during that span.

With the Angels facing elimination, Appier started Game 6, pitching four innings of shutout ball before yielding a two-run home run in the fifth inning, and leaving with a man on base who eventually scored via stolen base and wild pitch off Francisco Rodriguez, leaving him with an 11.37 ERA for the Series, which the Angels won in seven games.

In October 2005, Appier applied for reinstatement with Major League Baseball and signed a minor-league contract with the Seattle Mariners.