Perez apparently was injured before the game in the left-field bullpen while warming up starting pitcher Jonathan Sanchez before the Cactus League game against the Reds.
While the Royals were beating the Reds, 7-5, Perez was taken for an MRI to determine what might be wrong.
"I'm not going to overly concern myself until I hear exactly what the deal is with him," manager Ned Yost said after the game. "But honestly I'm a little bit concerned."
Initial details were so sketchy that Yost didn't know which knee Perez had injured. He was tended by head athletic trainer Nick Kenney.
"He was down there warming up Sanchez, getting ready to bring him in the game and, quite frankly, I really don't know what happened," Yost said.
"[Catching assistant] Jason Kendall came up and said, 'Boy, Salvy's limping a little bit coming in, he might have done something to his knee.' And I walked down and the trainers were looking at him."
Although the knee was sore, Perez could get into his crouch so he was permitted to catch the first inning, then was taken out of the game.
"You know a lot of it this time of year is precaution so I don't know the extent of it. I know it was sore and they wanted to get him out of there right away and find out what it was," Yost said.
If Perez is out for a while, the Royals will have to rely on backup Brayan Pena. They already have lost the third catcher on the Major League roster, Manny Pina, to knee surgery and he'll be out for the rest of Spring Training.
Perez was replaced on Tuesday by Triple-A catcher Cody Clark, who caught the rest of the game and contributed two hits to the Royals' 16-hit attack.
Perez, who was taken out before he batted, is hitless in 15 at-bats so far in the Cactus League.
On Feb. 27, the Royals signed Perez to a five-year contract that runs through 2016 with club options for each of the following three seasons, 2017-19. The contract guarantees Perez $7 million but could be worth a total of $26.75 million for the full eight years if all the options are exercised and all the incentives written into the deal are achieved.
Perez was called up from the Minors by the Royals on Aug. 10 and made an immediate impression with outstanding defensive skills and a .331 average in 39 games.