Anthony Rizzo out indefinitely

CHICAGO -- Cubs first baseman Anthony Rizzo is out indefinitely after an MRI Tuesday revealed a mild back strain.

The All-Star slugger, who is second in the National League with 30 home runs, has missed the past seven games with the injury and would have ended up on the disabled list if it weren't for the roster expanding in September.

"We're going to be cautious with him," Cubs president Theo Epstein said Tuesday. "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. Asked if the Cubs would shut Rizzo down with just a few weeks left in the season, Epstein said: "I think we'll see where we are down the road."

Rizzo is just happy to know there is something behind the pain he has felt in his back.

"Certain things I do I feel it. Twisting is when I feel it most," Rizzo said. "When I swing it hurts. It's kind of relieving that the MRI showed something so I'm not going crazy anymore. Hopefully in 10 (days) here I'll feel a lot better."

The 25-year-old first-time All-Star is batting .278 with 71 RBIs. Chris Valaika has been the primary starter at first in Rizzo's place.

"It's an opportunity for the guys here," Rizzo said. "My main concern is being full strength in the offseason, getting ready for next year. I want to play, but we all decided we're not going to go crazy and rush back and have it re-occur over and over. We want this to be the first and last time this happens."

The Cubs called up seven players from Triple-A Iowa on Tuesday: outfielder Junior Lake, catcher Rafael Lopez and pitchers Arodys Vizcaino, Brian Schlitter, Dan Straily, Zac Rosscup and Eric Jokisch.

ESPNChicago.com's Jesse Rogers contributed to this report.