After arriving to training camp in a hot-air balloon, NFL star Antonio Brown was keen to show the foot injury keeping him off the practice field was anything but hot air.

Signed from the Pittsburgh Steelers on a three-year $73 million contract, Brown is seen as the key to Jon Gruden's attempt to turn around the floundering Oakland Raiders.

But after his spectacular arrival, he's only taken part in 30 minutes of drills, prompting some to question his presence on the injury list.

Brown, who is headed for the Hall of Fame after racking up more than 11,000 receiving yards and 74 touchdowns in nine seasons with the Steelers, responded by posting a photograph of the blistering on his feet to Instagram.

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According to ESPN, Brown's foot injuries were the result of extreme frostbite caused by a cryotherapy machine.

He reportedly didn't wear the appropriate footwear when he used a cryotherapy machine in France last month.

The Athletic reported the 31-year-old visited a specialist last weekend and is not expected to be out for long. "We're all disappointed," Gruden said. "We'd like to get the party started."

"We wish that the healing process of whatever's going on be fast," quarterback Derek Carr said. "We want it to be fast, but at the same time you can't rush things for training camp when you know you have a season and, hopefully, a playoff run to think about."

Carr isn't concerned about finding a rhythm with his new top target because they've already spent plenty of time together during the off-season.

"We spent hours upon hours throwing," Carr said. "So there is chemistry, there is development. I'm used to throwing him the ball, so it won't be brand new when he shows up.

Antonio Brown signed from the Pittsburgh Steelers on a three-year $73 million contract. Photo / Getty Images
Antonio Brown signed from the Pittsburgh Steelers on a three-year $73 million contract. Photo / Getty Images

"It will be just more on him, and the details of the routes, getting out and doing it and correcting it on film and going and fixing it the next day. Those kind of things. But besides that, the timing aspect, the meat and potatoes, so to speak, of what we need is there."