The club hasn't confirmed the surgery, which Brown tweeted should be done within the week.
Machado was placed on the disabled list retroactive to Aug. 12 with a right knee sprain. Manager Buck Showalter told reporters today that Machado wasn't performing baseball activities and wouldn't be ready to come off the DL on Wednesday.
"No, that won't happen," Showalter said before today's 4-1 loss to the Cubs. "I'll probably try to bring some clarity and everything to where we are with that situation, try to gather exactly what he's being told, as far as when baseball activities would start and all that stuff. I'll try to clear that up for you probably tomorrow and get a more definitive idea."
Chris Davis made another start at third base today, with Steve Pearce starting at first. The Orioles could intensify their efforts to acquire an infielder who's passed through waivers.
Los Angeles orthopedic surgeon Dr. Neal ElAttrache likely would perform the surgery on Machado. He handled the procedure on Machado's left knee in October.
Brown also tweeted that a "small abnormality" found in the left knee also exists in the right knee, and it will be fixed during the surgery.
Machado missed the first month of the season while recovering from the first surgery. He's batting .278/.324/.431 with 14 doubles, 12 home runs and 32 RBIs in 82 games.
The Orioles are 20 games above .500 despite losing Machado and catcher Matt Wieters, who underwent ligament-reconstructive surgery on his right elbow in June.
Update: I received confirmation that Machado has made a "preliminary" decision to undergo knee surgery after consulting with doctors. Machado apparently doesn't believe the knee would heal sufficiently through rest and rehabilitation.
Executive vice president Dan Duquette will address the media at 11:30 a.m. Saturday.
The Orioles are 26-18 without Machado this season.