He returned to action on September 14, and went 6-for-27 (.269) in eight games; he hasn't played since September 24.
Now, with the Pirates' 2012 collapse complete, it's been revealed Walker's problems were more than just tightness:
Walker decides to shut it down for season; updated diagnosis: herniated disc.— Tom Singer (@Tom_Singer) September 28, 2012
Walker finishes his season with pretty good numbers, especially so if he had been battling these back troubles for much of the year: .280/.342/.426 with 14 home runs and 69 RBI in 129 games. Walker had been durable in 2011, playing in 159 games; the Pirates clearly missed his bat while he was out, as they went 4-11 and fell out of the NL wild-card race during that time frame.