Lopez thinks pitching in a 20-inning game this month is at least partly to blame for the injury, which the team expects will heal with rest.
Lopez said Monday he felt tightness in the elbow when sliding headfirst into second on a double during the first week of the season and figured he'd be able to play through it.
"Pitching obviously didn't help the situation," he said. "At the time I thought I could do it."
Lopez was batting .273 with a homer, four doubles and four RBIs in 44 at-bats, but started only two games after working an inning against the Mets in a 2-1 loss April 17 and won't throw again for two weeks.
Cardinals medical personnel weren't aware Lopez had been experiencing tightness until last weekend, and he didn't bring it up before heading to the mound for a scoreless 18th inning.
"I never say no to whatever the team needs. If I have one leg, I'll do it," Lopez said. "It was just tightness. It wasn't what it is now."
Manager Tony La Russa didn't second-guess himself for asking Lopez to pitch.
"I've got to put him out there," La Russa said. "But if you look at his throws [from the mound], they're less stressful than throwing from third base, second base and shortstop."
Outfielder Allen Craig, 1 for 18 in his first major league experience, was optioned to Memphis.
Jay, a second-round draft pick in 2006, is making his major league debut.