Assistant general manager Gord Ash said an MRI on Weeks showed a meniscus tear. Weeks would be eligible to return for the World Series if he can play through the injury. He will need surgery to correct the problem, which has not been scheduled.
Ray Durham was to start Game 4 against Philadelphia, which leads the NLDS series 2-1 heading into Sunday's game. Veteran Craig Counsell has extensive second base experience, while Escobar has primarily played shortstop.
Weeks appeared to jam his knee while batting in the second inning, when he was narrowly cut down at first after a hard-hit grounder to third. He left the field limping after getting tangled with Phillies first baseman Ryan Howard, but came out to play defense in the top of the third. He didn't bat in the bottom half, and was replaced by Counsell when the Brewers took the field for the fourth inning.
"He got caught up there with Howard at first base," Ash said. "If you watch the replay, it's not necessarily the action of tripping over Howard to some degree, it's the spinning around after he gets 10 feet past the bag."
Weeks finished the series 0-for-4 against Philadelphia.
Escobar was the staff's pick to replace Weeks even though he has a lack of big league experience. Considered the potential shortstop of the future, Escobar hit .328 with 34 stolen bases at Double-A Huntsville.
The September call-up has two hits in four major league at-bats over nine games.
"He can play all the infield positions," Ash said. "They feel like they can move him around enough to make him the guy they want to use right now."