After two seasons without a playoff berth, the Phillies on Thursday fired Kapler, whose team (81-81) underachieved even with the addition of big-money free agent Bryce Harper and whose nontraditional, analytical style irritated many of the franchise's passionate fans.
"Several years ago, I promised our loyal fans that I would do everything in my power to bring a world championship team to our city," Middleton said in a statement. "I will never waver from that commitment. ... I have decided that some changes are necessary to achieve our ultimate objective. Consequently, we will replace our manager.
"I am indebted to Gabe for the steadfast effort, energy and enthusiasm that he brought to our club, and we are unquestionably a better team and organization as a result of his contributions."
Kapler is the third consecutive Phillies manager to be fired after no more than two full seasons, joining Pete Mackanin and Ryne Sandberg. Middleton said general manager Matt Klentak will lead the team's search for a new manager.
The team also announced that pitching coach Chris Young, head athletic trainer Scott Sheridan and assistant athletic trainer Chris Mudd will not return. Hitting coach Charlie Manuel will return to his role as a senior adviser. The new manager will inherit the remainder of the coaching staff.
The Phillies have had internal conversations about Buck Showalter and Joe Girardi, among others, as potential replacements for Kapler, sources told ESPN's Buster Olney. Some executives have speculated that former Chicago Cubs manager Joe Maddon could also be a candidate, according to Olney.
Showalter used to work under Phillies president Andy MacPhail and with Klentak when all were with the Baltimore Orioles.
The Phillies are the eighth team seeking a new manager this offseason, joining the Angels, Cubs, Giants, Mets, Padres, Pirates and Royals.
"I have tremendous respect for this organization, this franchise and this city," Kapler said in a statement. "We came into 2019 with very high hopes. We fell short of those, and that responsibility lies with me. The next Phillies manager will inherit a team of talented, dedicated and committed players. There has been nothing more fulfilling in my professional career than the opportunity to work with the players on this team.
"... As I move on, I know that this organization is in a great spot and will see a lot of success going forward. My hope is that I helped contribute to a developing culture in the organization that flourishes in the years to come."
Rival executives have wondered if Kapler might emerge as a managerial candidate with the Giants because of his ties with San Francisco's Farhan Zaidi, according to Olney. Zaidi, the Giants' head of baseball operations, used to work with Kapler in the Los Angeles Dodgers' front office.
Philly took a gamble when it made Kapler the 54th manager in team history in November 2017, hoping a former big leaguer short on managerial experience -- he previously had managed only one season in the minors (2007) -- could lead the Phillies back to October baseball for the first time since 2011.
But the Kapler era in Philadelphia never took off.
In his first game, the Atlanta Braves rallied from a five-run deficit, winning on a three-run homer in the ninth inning. Kapler faced immediate scrutiny for lifting starter Aaron Nola with the Phillies up 5-0 and one out in the sixth inning. When Philly returned home after a season-opening 1-4 road trip, Kapler was booed resoundingly by Phillies fans.
A historic contract given to Harper and big trades for catcher J.T. Realmuto and infielder Jean Segura didn't help much in 2019. Although the Phillies spent much of April and May in first place, a seven-game losing skid in June stalled any momentum. And while they stayed in the National League wild-card race, they lost eight of nine in late September and ultimately were eliminated from the postseason by Harper's former team, the Nationals.
Injuries were a big reason the Phillies couldn't record their first winning record since 2011. They lost leadoff hitter Andrew McCutchen for the season in June, and six of their top seven relievers missed significant time. Free-agent addition David Robertson pitched just 6 innings, and Pat Neshek and Tommy Hunter threw a combined 23 innings. Also, starting center fielder Odubel Herrera played just 39 games before he was suspended for the rest of the season under Major League Baseball's domestic violence policy.
The Phillies finished fourth in the NL East -- 16 games behind the first-place Braves and eight games behind Milwaukee for the second wild-card spot. In 2018, the Phillies were third at 80-82, finishing 10 games behind the Braves.
"I want to thank Kap for his tireless commitment to the Phillies over the last two years," Klentak said in a statement. "When we hired Kap, it was our goal to develop a positive, forward-thinking and collaborative culture throughout the organization that would allow us to compete with the best teams in the league year in and year out.
"While we have fallen short in the win column for the last two years, I can confidently say that Kap's efforts have established a strong and sustainable foundation for this organization moving forward."
Kapler, 44, hit .268 in 12 major league seasons as an outfielder with the Boston Red Sox, Texas Rangers and four other clubs. Before managing the Phillies, he spent several years as the Dodgers' director of player development.
The Associated Press contributed to this report.
Kurkjian: Phillies want a 'veteran name' to replace Kapler
Tim Kurkjian lists Joe Maddon, Joe Girardi, Dusty Baker and Buck Showalter as possible candidates to replace Gabe Kapler as the Phillies manager.