Jackson was to take part in batting practice with the team Monday before their series opener against Boston.
Jackson was regarded as the best high school position player available in the draft. Jackson was a star at Rancho Bernardo High School near San Diego. He batted .400 with 11 home runs in his senior season after hitting 14 homers as a junior and 17 as a sophomore. His 45 career homers are tied for the most in CIF San Diego Section history.
The lingering question with Jackson will be his position after he spent most of his time in high school as a catcher. He's expected to start his career with the Mariners as an outfielder.