Arizona assigned Klassen to the AAA Tucson Sidewinders of the Pacific Coast League. Over the course of the 1998 season, he posted a .292 average with Tucson, tallying 10 homers, 47 RBI, and only 6 stolen bases over the course of 73 games. On July 4, Klassen made his Major League debut. Facing the Houston Astros, Klassen made four plate appearances, striking out once and drawing three walks. He was caught attempting to steal second base in the fifth inning. He made 29 appearances with the Diamondbacks in 1998, batting .194, with three home runs and 8 RBI. Klassen's first Major League home run came on July 10, as he hit a two–run shot off Cincinnati Reds relief pitcher Danny Graves.