Young told his social-media followers, "Thanks to the @packers for the opportunity great organization great team wonderful fans good luck this season."
The move, which was not immediately announced by the Packers, came as bit of a surprise since the team earlier in the week had cut Graham Harrell, Young's main rival for the job of backup to Aaron Rodgers.
Young, 30, was the third overall selection in the 2006 NFL draft and played for five years with Tennessee.
He last played for Philadelphia in 2011 and had been cut by Buffalo during training camp last summer.
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