Stewart signed a two-year $500,000 contract with the Athletics prior to the 1987 season. He won 20 games in 1987, while posting a 3.68 ERA and striking out 205 batters. He credited pitching coach Dave Duncan with helping improve his pitches. In 1988 he won his first eight games and claimed the Major League Baseball Pitcher of the Month Award. He went 21–12, 3.23 ERA while leading the American League (AL) with 14 complete games and innings pitched. Stewart started two games in the A's four-game sweep of the Boston Red Sox in the 1988 American League Championship Series. After the series, his manager Tony La Russa said he had "never met a player or person of higher quality." Stewart was the starting pitcher in Game one of the 1988 World Series against the Dodgers. Prior to the game, he told Dodgers' second baseman Steve Sax that he was going to hit him in the neck during the game. He came close, drilling Sax in the left shoulder and glaring at him. He allowed three runs in eight innings and was in line for the win before Kirk Gibson's walk-off homer off closer Dennis Eckersley gave the game to the Dodgers. Working on three days rest, Stewart wasn't as good in Game four, allowing four runs (only two earned) and leaving the game with one out in the sixth to pick up the loss. The Dodgers finished off their championship with a win in Game five.