ASHBURN, Va. -- The Washington Commanders fired defensive line coach Sam Mills on Tuesday, with coach Ron Rivera citing a "difference of philosophy."

Jeff Zgonina, who played one season with Mills' father in the NFL, will replace him. Zgonina had served as Mills' assistant the past two seasons.

Rivera didn't elaborate on what he meant by a philosophical difference, but said it was "just some observations. I just felt it was a change I needed to make."

Mills had been with Rivera since he was hired as Carolina's head coach in 2011. Mills was an assistant defensive line coach until December 2018, when Rivera elevated him to the main role after firing Brady Hoke. Mills then came to Washington after Rivera was hired in January 2020.

Mills' father, who died in 2005, was inducted into the NFL Hall of Fame this past weekend.

Rivera said his history with the Mills family made the decision "very difficult."

"I've known Sam a long time and he's a very good football coach," Rivera said. "I appreciate everything he's done. He helped us win a division title our first year and things got tough last year, but there were some things I felt I wanted to change."

Washington's defensive line underachieved last season amid questions about the group's chemistry and willingness to play within the system. That led to friction in a unit that had been accustomed to a different play style.

At times, Rivera questioned the football maturity level of some members of the group and fretted that it led to bouts of selfish play.

This past offseason, Washington released defensive tackle Matt Ioannidis and allowed Tim Settle to sign with Buffalo in free agency. The Commanders drafted Phidarian Mathis in the second round.

"It's sad," Washington defensive tackle Jon Allen said. "Me and Coach Sam were getting along well, we were really making some progress as a defensive unit. It's the NFL; people have to remember it's a great game but a terrible business."

Allen called the timing "surprising."

"One thing I've learned in the NFL is that nothing is unusual," he said. "You have to expect the unexpected and as soon as you get comfortable, the NFL will surprise you.

"But it's not my job to worry about that. My job is to play football. We have the utmost faith in Coach Rivera."

Rivera told the players the news in a meeting before practice. Brent Vieselmeyer, who is in his third year with the staff, will help with the defensive ends. Former Washington end Ryan Kerrigan, who has been shadowing the defensive line coaches and helping when needed, will be here throughout camp.

Hall of Fame defensive end Warren Sapp also was helping in practice Tuesday. He had worked with the defensive line for several days in the spring as well. Neither Sapp nor Kerrigan will assume a greater role, Rivera said.

Zgonina was San Francisco's defensive line coach in 2017 and 2018. After being drafted in the seventh round by Pittsburgh in 1993, Zgonina played 17 seasons with seven teams, winning a Super Bowl with the St. Louis Rams.

"It's always hard," Zgonina said. "He is a friend and always will be a friend," he said of Mills. "I've known him almost his whole life -- I played with his dad. But I have a job to do and he had a job to do and we just have to keep moving forward."