Once he became a professional, the hype McDonald endured in college molded into pressure and expectations. By September 1989, three months after he was drafted, he was in the majors. He spent the start of the 1990 season in the minors, joined the Orioles bullpen in July, and by the end of that month made his first major league start, pitching a shutout. McDonald won each of his first five starts, posting a 1.72 ERA. Despite the occasional rough outing thereafter, he finished the year with a 2.43 ERA in 21 games (15 starts).