Before he sat over the weekend, Molina started 79 of the team's first 85 games.
"I think we just have to be smart," general manager John Mozeliak said. "He wants in the lineup, and that's great, but we have to make sure that we give him some rest from time to time."
That meant lifting him after seven innings Tuesday, when the Cardinals had a comfortable lead before the Astros tacked on four ninth-inning runs to make it a 9-5 game. Molina went 0-for-2 with a walk and a run scored before Tony Cruz took over behind the plate.
Molina has been arguably the league's most valuable player over the first half, which underscores how important his on-field presence is for the Cardinals. He enters Tuesday leading the National League with a .346 batting average and ranked second with 33 multi-hit games.
With a third catcher Rob Johnson now on the roster, the Cardinals have more flexibility to give Molina days off, while he could still be used as a pinch-hitter, or even at first base.
"It's something we're going to try and stay on top of," manager Mike Matheny said. "There are days when we're going to have to get him some rest, but when he feels good, we're going to use him. I've made it clear that when it gets hot, which it is, we're going to be very careful. There will be more days [off]. That was going to happen regardless of whether his knee started bothering him or not."
Jenifer Langosch is a reporter for MLB.com. Read her blog, By Gosh, It's Langosch, and follow her on Twitter @LangoschMLB. This story was not subject to the approval of Major League Baseball or its clubs.