Beltran abruptly exited Game 3 in the first inning after he tweaked his knee stepping on first base after hitting into a 4-6-3 double play to end the frame.
The official diagnosis was a left knee strain.
"We'll know more [on Thursday], for sure," Cards manager Mike Matheny said. "But it's a left knee -- he had some issues on and off all season. On and off his whole career."
Beltran was not available to reporters because he had been sent to an off-site facility to undergo the MRI.
At least temporarily, the injury cost the Cardinals one of the greatest hitters in postseason history. Beltran entered the day with the best OPS (1.327), slugging percentage (.838) and batting average (.378) in Major League history among players with at least 100 postseason at-bats. He leads a trio of Hall of Famers; Lou Gehrig is second with a 1.214 OPS, Babe Ruth is second with a .744 slugging percentage and Paul Molitor is second with a .370 batting average.
Beltran is batting .400 this postseason, and his exit on Wednesday could not have been more abrupt.
"It's crazy," said Beltran's replacement, Matt Carpenter. "You don't expect things like that to happen, especially as sudden as it was, and, really, without any [warning] -- nobody knew. Next thing you know, he's out of the game. You just have to be ready, because that kind of stuff happens."
Said third baseman David Freese: "We knew we had Carpenter coming in, and hopefully he can help us out. But the type of guy [Beltran] is, and what he means to us and how much he wants to win, I think that's what everybody was thinking about."
Carpenter watched Beltran walk into the tunnel near the dugout to put his gear away, showing no signs of injury. A moment later, Matheny told Carpenter to grab his outfield glove and get ready.
In the bottom of the third inning, Carpenter delivered a go-ahead two-run home run, the difference in the Cards' 3-1 win.
"When guys get hurt, especially a guy like Carlos, sometimes that can be kind of a letdown from an energy standpoint, as a team," said Carpenter. "I wanted to do something that would bring some life back. So that at-bat was important for what was at stake -- a runner in scoring position -- but also to bring some life back to the dugout. I was glad to get it done."
If Beltran is out for the Cardinals' Game 4 matchup with Giants right-hander Tim Lincecum, Matheny's options for right field would include Carpenter, Skip Schumaker and Shane Robinson. Carpenter and Robinson have never faced Lincecum, but Schumaker has, going 7-for-22 (.318) with a home run and a double in 22 regular-season at-bats.
Beltran has a long history of knee issues, and the Cards cut back his playing time in order to help keep the veteran outfielder healthy when his knees bothered him in September.