The D-Backs have limped to a 59-81 record this year after back-to-back .500 seasons. The team has not determined whether or not manager Kirk Gibson will return next season, per Piecoro.
Towers was previously general manager of the San Diego Padres from 1995 to 2009, making him the fifth-longest-tenured active GM in the Major Leagues before his firing. With the D-Backs, he became a lightning rod for criticism from saber-minded fans, who questioned his desire to focus on acquiring players with "certain intangibles... gritty, grinder type. Hard-nosed."
The team is expected to make Towers' firing official on Friday, according to Piecoro. Tony La Russa was hired for the newly-created job of chief baseball officer in May, and La Russa has the "ultimate call" as to the job security of Towers and Gibson in Arizona.
Towers' firing marks the second general manager departure this season, a relative spike in front-office changes. The Padres relieved Josh Byrnes of his duties in June, which marked the first GM firing in Major League Baseball since the Houston Astros fired Ed Wade in November 2011.