Washington starter Jordan Zimmermann was pitching to Ryan Ludwick in the bottom of the seventh inning when Ramos was unable to snare the delivery. Cincinnati third baseman Todd Frazier took off for second base, and Ramos, while attempting to plant and pick up the ball, dropped to the grass.
"His cleat caught when he went for the ball, and he twisted his knee," Davey Johnson said. "His knee kind of buckled. He was in a lot of pain. I've got a bad feeling he tore something, but we'll just keep our fingers crossed."
Ramos, who will get an MRI, was replaced by Jesus Flores.
"Flo, he's a great catcher," Zimmermann said. "He'll step in and do great."
Ramos led off the fifth inning with his third home run of the season -- a 409-foot shot to straightaway center off Mat Latos -- that tied the game at 1. The potential loss of Ramos comes on the heels of Jayson Werth's broken wrist, which could sideline him for 12 weeks.
"We've got pretty good depth here, and pretty good talent here, but I think we've had enough," Johnson said. "Losing Willy, that's a big, big blow."
Tony Meale is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.