"This was a challenging day, a difficult thing to do," Luhnow said.
Porter, a Houston resident, had been the Astros' manager since the start of the 2013 season after serving as third-base coach of the Nationals. He went 110-190 in nearly two seasons on the job.
Tom Lawless was named interim manager through the remainder of the 2014 season. Adam Everett will join the Astros' coaching staff, replacing Trembley as bench coach. The remaining coaches will continue in their current roles.
Luhnow didn't get into any specifics about why he dismissed Porter.
"At the end of the day, it came down to me feeling like at this point going forward, a different leader in the clubhouse was in the best interest of the Astros," he said.
Luhnow did say the decision to dismiss Porter didn't have anything to do with the team's win-loss record. The Astros, after losing a club-record 111 games last year, are on target to be one of baseball's most improved teams at 59-79 entering Tuesday and are five games clear of the Rangers out of last place in the American League West. That's already their most wins since 2010.
"It was a difficult decision and one I didn't come to lightly," Luhnow said. "This is not regarding wins and losses at the big-league level. I take responsibility for the roster, I take responsibility for our baseball operations, which includes the staff and the clubhouse.
"I felt strongly that we need a new direction for the next phase of where we are for the Houston Astros. I do believe we've made a lot of progress in the last couple of years and we're moving towards our goal of being consistently competitive. As I honestly evaluated our goals going forward, we needed a change in leadership in the clubhouse."
Owner Jim Crane said in a statement: "This was not an easy decision to make. We wish Bo nothing but the best in the future. Jeff has my full support moving forward. Our goal to bring a championship to Houston remains."
The managerial search process, which will include executive advisor Nolan Ryan and special assistant to the general manager Craig Biggio, will begin immediately. Luhnow said he had been considering replace Porter for a few weeks and made the decision Sunday prior to informing him Monday.
In a statement to select members of the media, Porter wrote, in part: "I want to thank the Astros organization for giving me my first opportunity to manage Major League Baseball. During my time in Houston I dedicated myself to do everything I could to help this organization win, in the short-term and for the long-term.
"I am proud of what we were able to accomplish in Houston with an organization in transition. I'm gratified we were able to bring some excitement to this city as a result of our improvement from 2013 to 2014. I am enthusiastic about what the future will bring for me and my family. I look forward to my next opportunity in Major League Baseball. The valuable experience I gained with the Astros will be extremely beneficial in all my future endeavors."
The timing of the announcement will allow the Astros the month of September to conduct a managerial search as they did two years ago when Brad Mills was let go in August 2012. Luhnow didn't rule out interviewing some of those considered for the job two years ago, a list that includes Rays bench coach Dave Martinez and former Padres/Phillies manager Larry Bowa. Biggio could also get consideration.
"As we get to the next phase, we're looking for a collaborative environment, a unified environment across all aspects, not just in baseball operations but across the organization," Luhnow said. "We want that spirit of working together towards a common goal and the culture that's built around that to be as effective as it can be, and that's really the goal."
Ken Rosenthal of FOXSports.com reported Friday there was tension between Luhnow and Porter, citing sources that said Luhnow second-guessed Porter's in-game management. Porter didn't deny the report, except to say Friday he hadn't talked with Crane about his relationship with Luhnow as the story claimed.
Luhnow said Monday the report had "numerous inaccuracies."
"I don't tinker with lineups, and I don't tell people when to hit and run," he said. "That's not my job. That's the manager's job. There are times where I'm going to ask questions about it. That's natural, that's communication, and make sure we're all aligned and things are being done for the right reasons."
This season saw the Astros make strides on the field with the influx of veterans like pitchers Scott Feldman and Chad Qualls and outfielder Dexter Fowler, the arrival of top prospect George Springer and the emergence of pitchers Dallas Keuchel and Collin McHugh. All-Star second baseman Jose Altuve could win a batting title.
"It's definitely surprising," Keuchel said. "I know there was some stuff that came out earlier this week about the rift between the front office and Bo. Some of the guys have said it's none of our business and we're paid to play, and that's what we're going to do. We know it's a business and we hate it any time anybody is relieved of their duties."
Porter was unhappy when Mark Appel, the team's No. 1 Draft pick last year, came to Minute Maid Park earlier this year for an early morning bullpen session with pitching coach Brent Strom, but the players were indifferent despite reports to the contrary.
"I think it was blown out of proportion," Keuchel said.
Porter certainly had a unique style of leadership and wasn't afraid to let his passion show. He filled the walls of the clubhouse with motivational reminders and tried to create team-bonding exercises during Spring Training through games and players-only meetings he called a synergistic chemistry lab.
"We all know if you look up and down our roster and compared it to the A's or the Angels or something like that, Bo did a pretty good job with the cards he was dealt," Feldman said. "He had a really tough job, just with our low payroll [of $45 million] compared to other teams and, honestly, the talent level. We're building more for the future."
Lawless, who has close to 35 years of experience in baseball as a Major League player, Minor League manager and coach, had served as the manager for Triple-A Oklahoma City earlier this season while Tony DeFrancesco was on medical leave before returning to his role as a roving infield instructor.
Everett, who spent parts of 11 seasons in the Majors as a shortstop, including seven with the Astros (2001-07), rejoined the Astros organization as a Minor League infield instructor in 2013.
Lawless and Everett will address the media Tuesday prior to a game against the first-place Angels.
"We've got an environment with a lot of young players who are continuing to finish their development at the big leagues, and it's important to have a certain type of environment for those players to succeed," Luhnow said. "The clubhouse chemistry environment is a complicated formula and there's no one thing that you can look at and say, 'That's what you need,' but you need to have a feeling that we're all aligned, that the leadership is all aligned and working towards the same goal."