Alou left Wednesday night's game against the Atlanta Braves with the calf injury, and had an MRI exam Thursday in New York. Alou, hitting .340, has played in only 14 games after recovering from hernia surgery.
The Mets selected the contract of catcher-first baseman Raul Casanova from Triple-A New Orleans.
Casanova, who hit .283 with one homer in 46 at-bats with the Mets earlier this season, joined the team for Thursday night's game against the Braves.