Anthony Rizzo is out indefinitely after an MRI revealed a mild back strain, the Chicago Cubs have announced.

Rizzo is second in the National League in home runs (30) and has missed the last seven games with the injury.

"We're going to be cautious with him," Theo Epstein said. "It's the type of thing where if it was before expanded rosters he probably would be a DL. With the expanded rosters in September there is no need to DL him. We're going to be cautious and make sure we knock it out. It's nothing chronic, nothing that will bother him long term, but it's just something we don't want to rush back from."

Rizzo tweaked his back on a swing last week in Cincinnati.