Bench coach Trent Jewett will run the team in McClendon's absence. The first-year Mariners skipper has guided the club to a 72-60 record going into Friday's series against the National League East-leading Nationals.
Jewett, 50, is in his seventh season as a Major League coach, and first with the Mariners. He spent the past three seasons with the Nationals, first as their first-base coach in 2011-12, and then as third-base coach last year.
Jewett spent 17 seasons managing in the Minor Leagues, including 12 seasons at the Triple-A level, and has a career record of 1,178-1,166.