David Montgomery, the Phillies' president and CEO since 1997, will take a medical leave of absence amid a tumultuous time for the franchise. Former general manager Pat Gillick, the architect of the 2008 world-champion Phillies, will assume Montgomery's duties, the team said Thursday.

Montgomery, 68, underwent surgery to remove cancer from his jawbone in May. He has maintained an active presence at Citizens Bank Park ever since - even during chemotherapy and radiation treatments - and helped greet Taney Little League players during a pregame ceremony Wednesday at the ballpark.

The leadership distress has exacerbated the turbulence for a franchise that will have spent more than $500 million in player salaries over a three-year period without a postseason berth. The future of general manager Ruben Amaro Jr., whose contract runs through 2015, is a constant source of public friction.

The Phillies said Montgomery will return to his duties as the team's chief executive when he is "fully recovered." A team source said this decision did not stem from an ownership shake-up or subterfuge. But Montgomery's stepping aside could soon create permanent change.

In the meantime, the 77-year-old Gillick will handle the day-to-day operations of the entire organization.

"I have the highest regard for David Montgomery, as does everyone in our industry," Gillick said in a statement. "I am glad to be of assistance to the Phillies."

Gillick was not immediately available for comment.

Respect for Montgomery is universal in the game. He served on a seven-man committee appointed by outgoing Major League Baseball commissioner Bud Selig earlier this year to find his replacement, Rob Manfred. Selig said during a May visit to Philadelphia that Montgomery "is clearly one of my favorite people in baseball and has been for many years."

Despite his frail state, Montgomery traveled to Washington with the team at the July 31 trade deadline. He also made trips in conjunction with Selig's committee.

Montgomery, who grew up in Roxborough and attended Penn Charter, started with the Phillies as a ticket salesman in 1971. He has operated as the team's top executive since 1997 with complete management authority from the silent ownership group. Montgomery acquired a small share of team ownership in 1994.

"He's a guy who, in a lot of ways, is our biggest fan," Phillies manager Ryne Sandberg said in May. "From my experience here the last four years, he's the guy who sets the tone as far as it being a family-oriented organization - first-class, the right way. He's the leader of the organization. . . . It all starts with David Montgomery."

Gillick was inducted into baseball's Hall of Fame in 2011. He served as senior adviser to the president and general manager until Thursday, and held a firm place in Amaro's inner circle. He now becomes Amaro's boss.

Amaro was one of Gillick's many proteges and succeeded him as general manager after the 2008 season. Montgomery, with input from Gillick, chose Amaro over Mike Arbuckle, another longtime Phillies executive.

The team's vice presidents will report to Gillick, whose extensive experience is in baseball operations. He could delegate most business matters to various officials, including Mike Stiles, a senior vice president for administration and operations. Stiles, a former United States attorney, was a classmate of Montgomery's at the University of Pennsylvania.

In a June interview with The Inquirer, Montgomery endorsed Amaro as general manager despite the team's continued failures.

"I think we have pretty good people doing these jobs," Montgomery said. "We saw, over a long period, pretty good success with this group of people. Obviously, Ruben is part of that group."

Last week, Montgomery spoke to a group of fans at Citizens Bank Park and reaffirmed his faith in Amaro's command.