After losing at WrestleMania 30 on Sunday night, The Undertaker was rushed to hospital with a head injury.
The wrestler suffered a "severe concussion", WWE has confirmed on their official website.
Details of how exactly The Undertaker sustained his injury have not been revealed.
"Following last's night historic match with Brock Lesnar, The Undertaker was immediately taken to Ochsner Medical Center," said a WWE statement.
"After a CT scan and other medical testing, he was diagnosed with a severe concussion and was kept overnight for further evaluation."
It added: "He was discharged early Monday afternoon."
For decades The Undertaker had an unbeaten WrestleMania run and a winning streak of 21-0.
He had previously beaten the likes of Triple H, Shawn Michaels and his brother Kane in earlier WrestleMania events.
This year he took on Brock Lesnar and, for the first time, was defeated - leaving many fans shocked and the news trending on social media.
Last month Newsbeat spoke with WWE star Fandango, who has himself suffered a severe concussion.
He said WWE "took great care" of him after his injury and that athletes and wrestlers in the US were helped by the research carried out by WWE.
Chris Nowinski, a former WWE star and American footballer, said young people in the UK aren't taught enough about head injuries and more needs to be done to educate them.
He retired after being diagnosed with post-concussion syndrome and has made a film highlighting the dangers.
WrestleMania broke the Mercedes-Benz Superdome record for the New Orleans venue's "highest grossing entertainment event, grossing $10.9m [£6.6m]".
According to official figures "attendance was a sold-out crowd of 75,167, with guests from all 50 [US] states and 37 countries".
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