"(Fox) is being held at the hospital for testing and further observation," the Broncos said in a statement.
"Our medical staff is currently in contact with Coach Fox, his family and the doctors who are treating him. Additional information will be provided as it becomes available."
The Broncos are on a bye this week.
The Dallas Cowboys placed veteran guard Brian Waters on injured reserve.
Waters suffered a triceps injury against Detroit last week and reportedly was hoping to avoid surgery, but the Cowboys need an extra cornerback with Morris Claiborne missing due to a hamstring injury.
The other cornerbacks on the 53-man roster are Brandon Carr, Orlando Scandrick and B.W. Webb.
Waters, a 13-year NFL veteran, had started five games since signing with the Cowboys on Sept. 4. Mackenzy Bernadeau will replace Waters in the starting lineup.
The Chicago Bears agreed to a one-year deal with defensive lineman Jay Ratliff.
Ratliff was released by the Dallas Cowboys two weeks ago after failing a physical.
He is reportedly two to four weeks away from being ready to play. He has been out for almost a year with a groin injury that led to sports hernia surgery in December. He also faces drunk driving charges stemming from an arrest six weeks after then-Cowboys backup Josh Brent's alcohol-related crash that killed teammate Jerry Brown Jr. Ratliff's trial is scheduled for February.
The NFL Players Association is investigating the Miami Dolphins' team culture after offensive tackle Jonathan Martin left the franchise.
Martin, the Dolphins' second-round draft pick in 2012 out of Stanford, has not been with the Dolphins since a Monday incident in which players jokingly refused to sit with him in the team's cafeteria.
Martin has reportedly been the subject of teasing and bullying by teammates in the locker room since 2012.
The NFLPA is investigating the possible role of Richie Incognito, who told NFL.com he was in contact with Martin on Friday after he reached out to him to see how he was doing.
"Yeah, I'm good man," Martin wrote during their conversation that lasted 19 text messages.
"It's insane bro, but just know I don't blame you guys at all. It's just the culture around football, and the locker room got to me a little."
One Dolphins official who did not want to be identified told ESPN Incognito has been a "model citizen" and is puzzled by the allegations of Incognito's involvement.
However, other sources told ESPN that Incognito has been involved in the alleged harassment of Martin going back to 2012. One of the sources said the emotional issues Martin has relate directly to the harassment.
An NFL spokesman said the league has contacted the Dolphins since Martin left the team and is monitoring the situation.
The players union said it is investigating the team's workplace environment and players allegedly involved, including Martin and Incognito.
NFLPA attorneys have already gathered details that identify Incognito prominently as one of the offenders, a source told ESPN.
The New England Patriots placed starting defensive tackle Tommy Kelly on season-ending injured reserve.
Kelly injured his right knee against the Cincinnati Bengals on Oct. 6.
The New Orleans Saints activated linebacker Jonathan Vilma to their 53-man roster.
Vilma was placed on injured reserve/eligible to return during the preseason after he had arthroscopic knee surgery. He began practicing last week.
Chicago Bears cornerback Charles Tillman was declared probable for Monday night's game against the Green Bay Packers.
The Chicago Tribune reported Tillman showed no signs of limitation with his injured right knee after practice Saturday.
Tillman has had inflammation in the knee since September.
Carolina Panthers running back Jonathan Stewart was activated from the physically unable to perform list.
Coach Ron Rivera said after Friday's practice that he would be officially placed on the active roster.