The Giants announced Thursday that Posey was on the disabled list with the injury, a night after he was crushed by Cousins while trying to block the winning run in the 12th inning against the Marlins.
He also was scheduled to have an MRI.
Posey has been San Francisco's best cleanup hitter this season, a team leader in the clubhouse and key cog behind the plate for one of the best rotations in baseball. He was batting .284 with four home runs and 21 RBIs, just finding his groove in the midst of a 13-game hitting streak.
His loss will be an enormous obstacle for the Giants to overcome if they want to repeat as champs. Backup Eli Whiteside will be Posey's immediate replacement.
The injury came after San Francisco's spirited four-run rally in the ninth to force extra innings.
The play started when Emilio Bonifacio hit a shallow fly ball to right-center off Guillermo Mota for the second out in the 12th inning. Cousins tagged from third base on the sacrifice fly, beating the throw from Nate Schierholtz and lowering his shoulder to slam into Posey for a clean hit on the reigning NL Rookie of the Year.
Cousins was safe as Posey never could quite corral the ball, giving Florida a 7-6 victory. Cousins, who went to the University San Francisco, lives in the Bay Area and had almost a dozen friends and family in attendance, apologized repeatedly for injuring Posey. But he believes — as most Giants also reiterated — that he made a clean baseball play.
"I'm not trying to end anybody's season or anything like that. I just was trying to play hard and score the go-ahead run," Cousins said after the game.
Posey laid in the dirt around home plate, dazed, writhing in pain and curling up in a ball. After several minutes of stunned silence at AT&T Park, fans began chanting "Posey! Posey!" as he was helped off the field by two team trainers holding his left leg.
Posey had already taken several hard foul tips off his mask and legs this season, even leaving one game for precautionary reasons to make sure he didn't have a concussion. Some observers have argued a slugger of his caliber shouldn't be behind the plate, where hits can be routine, and this injury surely won't do much to quiet that sentiment.
Posey himself has always shaken off those remarks, saying he was born to play catcher and loves his position.