"The next couple days will help us understand kind of what we're dealing with," manager Mike Matheny said.
Fowler ran, leaped and hit the wall with the knee before falling backward. He was down on his back on the warning track as the ball bounced away, and Adam Frazier ended up rounding the bases for an inside-the-park home run that gave the Pirates a 3-2 lead. The Cardinals later came back to earn a 4-3 victory on Saturday.
"Any time you see a teammate look like he seriously hurt himself, you hate to see that," third baseman Matt Carpenter said. "It ended up being a big run, but you care more about your teammate than the run at the time."
Matheny expressed relief that Fowler's injury wasn't worse.
"When I saw him laying on the track, I didn't know what had happened," Matheny said. "When he got up, it didn't look good, either."
Fowler was the Cardinals' big free-agent signing in the offseason, signing a five-year deal worth $82 million. But the veteran has been hampered by various maladies this season. He has missed time with an illness, a left hip issue, a stiff back, a strained left wrist, a right heel spur and a strained right shoulder. He's been on the disabled list twice, for the wrist and heel injuries.
Fowler, who is hitting .255 with 15 homers and 55 RBIs, will have to deal with this latest setback as the Cardinals continue to make a push for a playoff spot.
Jenifer Langosch has covered the Cardinals for MLB.com since 2012, and previously covered the Pirates from 2007-11. Read her blog, By Gosh, It's Langosch, follow her on Twitter @LangoschMLB, like her Facebook page Jenifer Langosch for Cardinals.com and listen to her podcast. This story was not subject to the approval of Major League Baseball or its clubs.