Vina suffered the injury while running to first base in Sunday's 8-7 loss to the Pittsburgh Pirates. An MRI on Monday revealed a complete tear of the medial hamstring tendon on the inside of the thigh.
"He's looking at several weeks of rehabilitation," said Cardinals team physician Dr. George Paletta. "Realistically, he's looking at 6-to-12 weeks. He will be re-evaluated in the next 48 hours. At that point, we will decide on surgery or rehabilitation."
The Cardinals did not immediately place Vina on the disabled list. Miguel Cairo started at second in Monday's 10-5 win over Houston. He went 2-for-5 and scored twice.
"Everything was starting to come together," said Vina, who had batted .345 over his previous 14 games. "It's just bad luck. When I fell on the ground, I couldn't control my leg, so I knew it was bad. I tried to put myself up and I couldn't."
The two-time Gold Glove winner is hitting .262 this season. In May, he was batting .310, with 11 extra-base hits and a .529 slugging average.
"That's a real tough break for him and us," said St. Louis Manager Tony La Russa. "He was looking so good at the plate. When you lose a guy, you're reminded of his value. People talk about Fernando struggling at the top of the lineup, but he's the best guy we have, and he's proven he's capable of igniting us."