Clik here to view.

The cursed season of injuries that has been 2013 reared its ugly head again in Baltimore after Brett Gardner left the game in the first inning. He has been diagnosed with a left oblique strain that could put the rest of his season in jeopardy.
An oblique strain takes a month to recover from, usually. Might put Brett Gardner on ice for a while.
— Andy McCullough (@McCulloughSL) September 13, 2013
With the Yankees within striking distance of a wild card spot, losing one of their better hitters could have a huge impact. A Gardnerless lineup would involve Ichiro Suzuki likely starting every day and leading off with his sub-80 wRC+ with every game being close to must-win. That's not a scenario the Yankees want, but there are very few options at this point. The lineup is already dramatically shortened with Brendan Ryan and Chris Stewart at the bottom; adding Ichiro to that would only serve to shorten it even more.
There is no timetable yet for Gardner's return. The best case scenario might be for it to be extremely mild and only cost him a week or so. Everyone will likely know more after the game, so keep your fingers crossed until then.