Molina has been dealing with soreness in his knee since late August, when he suffered an injury in a game against the Nationals. He sat out two games immediately after the injury, but has played more often than not in the days since. He sat out Sunday's game at home, then played on Monday and Tuesday before the persistent pain led the club to send him home to be checked out. Matt Pagnozzi started in Molina's stead on Wednesday night.
Molina has consistently downplayed the seriousness of the issue, but it has lasted long enough to be of some concern.
"We've been watching it closely," manager Tony La Russa said. "For him to agree to get it checked, there's more soreness in it."
La Russa did not dismiss the possibility that Molina could be shut down for the season if and when the Cardinals are eliminated from playoff contention.
"We're going to do whatever [the medical staff] decides is best," La Russa said. "Could that be an alternative? Sure."
Molina, 28, is batting .262 with a .329 on-base percentage and a .342 slugging percentage. He leads the National League in games and starts, and has caught over 100 more innings than any other catcher in the league.
Matthew Leach is a reporter for MLB.com. Read his blog, Obviously, You're Not a Golfer and follow him on Twitter at @MatthewHLeach. This story was not subject to the approval of Major League Baseball or its clubs.