Brown was hospitalized after air entered his bloodstream during a pregame IV in the team's locker room at FirstEnergy Stadium and he required immediate medical attention, a source confirmed to ESPN. It was first reported by NFL Network.
Brown's release was first reported by the Las Vegas Review-Journal.
"Very glad to report he is up and seems to be doing well," Raiders coach Jon Gruden said Monday in his weekly Zoom media conference. "We're awaiting the results of some of the extensive tests that they did today and yesterday. I don't have anything to report other than he had an issue.
"I want to really thank our medical staff, what a job they did in an emergency situation, and we're just happy he's up and around, and once we get the official word as to what's wrong with Trent, we'll let you know."
Brown, who signed a four-year, $66 million contract with the Raiders in 2019, had just come off the team's reserve/COVID-19 list on Friday and was slated to start for Las Vegas against the Browns.
Raiders quarterback Derek Carr described a scary scene in the pregame locker room.
"Honestly, I'm human, I got a little scared for Trent," Carr said after the Raiders' 16-6 win over the Browns. "I was like, 'Man, I hope he's OK.' We had no clue what was happening.
"All of a sudden, we come in and they're wheeling him out and I'm like, 'Bro, what is going on?' It was crazy, and so we prayed. As a team we prayed for him, we made sure he was OK, his family, because I'm sure they were scared to death."
Gruden said the Raiders were "all shaken up pretty good" before kickoff.
"You get shook up when you see one of your own go down and you don't know what's wrong with him," Gruden said. "But we were able to assure our players he was in good hands and his signs were vital and he was doing good. We said a prayer for Trent before the game, and we went out and tried to win a game for him. It's something that's on our mind and weighing heavily on us right now. We hope he's OK."
A pectoral injury ended Brown's season prematurely last year, and a calf issue limited him in training camp and to three snaps in the season opener at Carolina. He has played in only one other game this season -- at Kansas City on Oct. 11 -- after testing positive for COVID-19 two weeks ago.
Gruden was asked about Brown potentially playing for the Raiders (4-3) this weekend at the Los Angeles Chargers (2-5).
"We don't know, really, anything about Trent's status," Gruden said. "The most important thing is that we find out what went wrong. Our prayers are with Trent, certainly, and he's in the best possible hands he can be in right now. But his availability and his status is not up for discussion right now, as far as we're concerned."
ESPN's Paul Gutierrez contributed to this report.