Manuel, 69, was in his ninth season managing the Phillies, after having guided the team to the 2008 World Series title and 2009 National League pennant. He marked his 1,000th managerial victory Monday.
But after winning 102 games in 2011, Philadelphia dropped to an 81-81 team last year and was 53-67 this season when Manuel was fired Friday.
"It came down to a few things," Phillies General Manager Ruben Amaro said. "One, I wanted to make sure that if the decision was made Charlie would not be back, he should know. I didn't see any reason we should drag it out and let him sit for the next 40 games knowing he wouldn't be manager beyond this season. I didn't think that was fair to him.
"Additionally, it gives a chance to see what we have in Ryne Sandberg and how he can handle managing the Philadelphia Phillies."
Sandberg, 53, was Manuel's third base coach and has been given an interim manager title for the rest of this season. He was seen as the most likely replacement for Manuel since he joined the organization last off-season.
It is the first major-league managerial post for Sandberg, who has been a minor league manager for six seasons. He was voted to the Baseball Hall of Fame after a playing career that began with 13 games for the Phillies in 1981 and included 15 seasons and 2,151 games with the Chicago Cubs.
Philadelphia was 780-636 under Manuel, who owns the record for most victories by a Phillies manager. The team won five straight NL East titles from 2007-11 with Manuel as manager.
Manuel managed the Cleveland Indians for three seasons (2000-02) building a 220-190 record. His overall managing record is 1,000-826.