"Not good," said Padres manager Bud Black after the game when asked about Grandal's knee.
Grandal will be placed on the 15-day disabled list on Sunday. The team will add catcher Rene Rivera, who was playing at Triple-A Tucson, in time for Sunday's game.
As for Grandal, he will get an X-ray on the knee on Sunday. He will then fly to San Diego after the game with the team and have an MRI on Monday. If the knee requires surgery, then Grandal's season is likely over.
"It was one of those rare plays where you've got to keep your foot on the plate if you're going to get an out," said Grandal, who was using crutches after the game. "It happens."
With the bases loaded in a scoreless game, the Nationals' Ian Desmond grounded a ball at first baseman Jesus Guzman, who threw home with the hopes of getting a force on Rendon, who was running down the line on the play.
The ball arrived before Rendon did and Grandal reached to his right, slightly toward the first-base line. Rendon's lead leg appeared to twist Grandal's trailing leg, which remained on the plate. Rendon was called out on the play.
"I thought I'd have time to get my foot out," Grandal said. "I didn't really expect the impact. I knew that something was wrong. You just don't get knee pain for no reason."
Grandal remained on the ground for nearly two minutes as he received attention from the training staff. He eventually left the field with the assistance of two teammates, though he didn't put any weight on his right leg.
Grandal -- who struck out in his only at-bat and is hitting .216 -- was replaced behind the plate by Nick Hundley, who figures to get most of the playing time moving forward.
The Nationals went on to win the game, 5-4, extending the Padres' losing streak to a season-long eight games.
"Unfortunate situation," said Padres pitcher Jason Marquis, who started Saturday. "Obviously he made a great play picking the ball out, and I guess you expose yourself sometimes as a catcher. By the way he was yelling and grabbing his leg, I knew it wasn't good."
Grandal sat out the first 50 games of the regular season while serving a suspension after testing positive for testosterone, a violation of Major League Baseball's Joint Drug Prevention and Treatment Program. He had one home run and nine RBIs in 28 games.
"You obviously never want anything bad to happen to you, but a lot of catchers have gone through this. It goes with the job," Grandal said. "We'll do rehab if we need to. We'll do surgery if we need to."
Rivera was hitting .343 with five home runs and 38 RBIs at Tucson. He has appeared in 98 big leagues games with the Mariners (2004-06) and Twins (2011).
Rivera spent all of Spring Training with the Padres, so he's familiar with the pitching staff.
"He knows our staff and our guys know Rene," Black said. "He's been around."
The Padres will need to make a move to create a spot on the 40-man roster for Rivera. That will likely be announced on Sunday.