Manuel confirmed the news to MLB.com before Monday's game against the Astros at Citizens Bank Park.
The 65-year-old Manuel said he made an appearance about a week ago for Nutrisystem in Horsham and then posed for a number of print ads.
"Yep, I went there recently on an off-day," Manuel said. "It was good for me."
Manuel said he has lost about 50 pounds thanks to Nutrisystem. In Spring Training, he weighed in at approximately 280 pounds, and he has taken off 48 pounds since that point.
SportsBusiness Journal notes that Nutrisystem has endorsement deals with several former NFL players such as Cris Carter, Lawrence Taylor and Dan Marino.
Manuel said he expects to be asked to be part of some television spots in the offseason.
"I hope that's a while from now," Manuel joked, referring to the Phillies getting ready to possibly defend their World Series title.
Andy Jasner is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.