Raiders receiver Antonio Brown’s mysterious foot injury is the result of extreme frostbite caused by a cryotherapy machine, according to reports Wednesday.

Apparently the seven-time Pro Bowler and four-time All-Pro in his first training camp with the Raiders, entered the machine without proper footwear.

Full-body cryotherapy is a popular recovery method used by athletes. Basically you stand in a cryotherapy chamber for roughly three minutes as temperatures drop as low as minus-11.

Proper footwear is a must. Or should be.

Brown wouldn’t be the first athlete to have a mishap with cryotherapy. Last year two Missouri State basketball players developed foot blisters after a cryotherapy session.

Tender tootsies have been a source of frustration for coach Jon Gruden, who’s counting on the former Steeler to jump-start the team’s offense.

Gruden told ESPN that Brown was “still collecting information” on the injury, and that the team would know more in the next few days. It was reported last week that the injury isn’t expected to be long-term.

Brown averaged 1,245 receiving yards and eight touchdown catches the last nine seasons.

Bosa exits practice: Rookie defensive end Nick Bosa underwent an MRI for a possible right ankle injury after leaving 49ers practice early.

With about 45 minutes left in the practice Wednesday, Bosa appeared to get tangled up with multiple teammates on a running play off the right side and stayed down on the field after the play ended.

The team’s medical staff rushed to his side and examined the lower part of his right leg and ankle.

Bosa was able to stand and walk off the field with two trainers. He slowly headed into Levi’s Stadium, without assistance, for further examination.