Starks was a mainstay of the Steelers' offensive line for several years before a neck injury stopped him down the stretch in 2010. There have recently been whispers that he currently weights "in the high 300s," so it's possible the Steelers are unhappy with his conditioning or with the way he recovered from his injury.
Starks' departure would leave a thin offensive line even thinner (speaking figuratively, but I suppose also literally). This move could signal that the Steelers think they can convince Willie Colon to return. Or it might just mean that they think Starks is mostly done. The Steelers have never been slow to cut veterans and replace them with younger players. It's possible they really like second-round draft pick Marcus Gilbert and think he could have a key role on the offensive line this year. We'll have to see.
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