Ryan, who is hitting .281 and has played outstanding defense recently, was batting in the bottom of the second against the Dodgers' Hiroki Kuroda when he fouled a ball off the inside of his left ankle. He stayed in the game after a lengthy visit from head athletic trainer Barry Weinberg and grounded to shortstop to end his at-bat.
But when Ryan tried to run to first base, he limped in obvious pain and was helped to the clubhouse and removed from the game.
X-rays taken during the game were negative, but Ryan thinks he will miss at least a couple of games.
"I'm kind of concerned that something more serious might be wrong," Ryan said. "I haven't had anything swell up that quick. I got real, real tight and couldn't really hinge my foot. That's why I didn't make it to first real well.
"Within a couple of minutes, I couldn't put an ounce of weight on it. It looked gross."
Ryan was given crutches and fitted for a boot that stretched up near his knee to try to keep the area compressed and eliminate some of the swelling.
"It's a bruise, but I mean, there's some bleeding in there because I crushed a vein or something," Ryan said. "I hit it pretty hard. It was scary swollen, so I'm just glad to know it's nothing serious.
"We will assess it again tomorrow. It was about the size of a tennis ball. It was kind of scary there for a second."
The 27-year-old Ryan is in his third season with the Cardinals but his first as an everyday player. He won the shortstop job early in the season and has improved his play as the season has progressed. He went 4-for-4 at the plate on Monday night and made several highlight-reel plays defensively on Tuesday and Wednesday.
The newly acquired Julio Lugo appears to be in line for most of the playing time while Ryan is out, but the Cardinals could activate Khalil Greene from the disabled list to compete for some of the playing time as well.
The Cardinals are off on Monday before starting a quick two-game series in New York on Tuesday night, and barring a super-quick recovery, that appears to be the soonest Ryan could return to action.
"It's going to be pretty bad tomorrow, so hopefully Saturday, Sunday, it may even be Tuesday," Ryan said. "It looked like I might of burst something in there. It was just so swollen, but we did the X-rays right away and that was encouraging.
"It swelled up pretty good, pretty quick. I wish it hit my shin guard. It hit about an inch away. It's a nice bruise. We will see how I feel tomorrow."
B.J. Rains is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.