"Trent Brown felt ill before the game, resulting in his deactivation," the Raiders said in a statement. "He did not exhibit any COVID-related symptoms."
Brown, who signed a four-year, $66 million contract with the Raiders in 2019, tested positive for COVID-19 less than two weeks ago.
Raiders coach Jon Gruden said after Sunday's game that Brown was slated to start. A source told ESPN's Adam Schefter that Brown was feeling better on Sunday evening.
"We're just going to say that he got ill here in the locker room and he's being evaluated right now," Gruden said. "We'll update you as soon as we something. But I'm told everything is OK. We were expecting Trent to play today; he had a great practice on Friday."
Raiders quarterback Derek Carr described a scary scene before the game and said he was concerned for Brown.
"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," Carr said. "As a team we prayed for him, we made sure he was OK, his family, because I'm sure they were scared to death."
"We could write a pretty good book in just my seven years here," Carr added. "There's been some crazy stuff that has happened. And today was another one of those things."
Brown was limited to three snaps in the season opener at Carolina because of a calf injury, then he played a full game at Kansas City on Oct. 11. He has played 10-plus snaps in just 11 of 22 games since signing with the Raiders.
Ahead of the team's Week 7 game against the Tampa Bay Buccaneers, because Brown was not wearing his tracker at the team facility before testing positive, the four other starters on the offensive line had to self-quarantine and were not eligible to play until hours before the Raiders kicked off last Sunday against the Buccaneers.