The 29-year-old Frenchman suffered a partially torn medial meniscus in the knee in a 100-79 pre-season home victory Thursday over Philadelphia.
The Wizards said Mahinmi underwent successful surgery Friday to repair the injury but was expected to miss about four to six weeks -- keeping the reserve center benched for the first month of the NBA campaign.
Mahinmi had signed a four-year deal worth $64 million with the Wizards in July.
"It's unfortunate when any guy gets hurt, but that's part of the game," Wizards coach Scott Brooks said. "He was really starting to play well with his defensive rim protection."
Mahinmi produced career highs with 9.3 points, 7.1 rebounds and 71 starts last season with the Indiana Pacers. He had only managed averages of 5.1 points and 4.3 rebounds over eight prior NBA seasons with San Antonio, Dallas and Indiana.