Brown was largely restricted to watching practice in the opening episode of 'Hard Knocks, ' sidelined by a mysterious foot injury at Raiders training camp. ESPN's source also said that Brown did not wear proper footwear while using the machine last month in France.
The issues that are keeping the star wideout from the practice field were thought to be blisters, with Brown posting a photo on social media on Monday showing skin peeling from his feet.
They just traded for one of the NFL's most popular and divisive players with Brown, who is now seriously missing time after getting "extreme frostbite" in his feet from a cryotherapy mishap.
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Raiders coach Jon Gruden said he is "gathering information" about the injury but could not say when Brown would return to action.
It all could make for a compelling storyline on HBO's Hard Knocks series, which is tracking the Raiders during camp.
The 31-year-old became one of the highest-paid wide receivers in the National Football League when he joined Oakland from Pittsburgh Steelers in March on a three-year deal worth more than $50m (£41m) per season. The Raiders have not offered many comments on what's ailing Brown or how long he could be out. The Central Michigan product amassed 104 passes for 1,297 yards last season as a member of the Pittsburgh Steelers.