Jim Slaton, the club's pitching coordinator at the Camelback Ranch Minor League complex, will take over on an interim basis.
In 2008, Howell missed a couple weeks of the season due to complications from diabetes that required the amputation of one of his toes.
In other medical news, struggling reliever Todd Coffey, who received treatment all week for right knee soreness, was placed on the disabled list Saturday to make room for starter Ted Lilly, who allowed one unearned run in the Dodgers' 6-1 victory over the Padres. Coffey has insisted several times this week that he's fine physically, despite his ineffectiveness.
Josh Lindblom made the club in Lilly's place, but he's already become too valuable for the club to send him back to the Minor Leagues.
Ken Gurnick is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.