Del Rio went 68-71 from 2003-11 as head coach of the Jacksonville Jaguars before joining Fox's staff last season.
Del Rio will continue to serve as Denver's defensive coordinator.
The Broncos are ranked 24th in the NFL in defense this season, but they were missing star linebacker Von Miller and cornerback Champ Bailey for much of the first half of the season.
Fox needs aortic valve replacement surgery and will miss several weeks. He was taken to a hospital near his Charlotte, N.C., home Saturday when he began feeling dizzy while playing golf during the Broncos' bye week.
The Broncos (7-1) play at San Diego on Sunday
More Sports Headlines
An open letter to new M's GM Jerry Dipoto Goff throws 4 TD passes as Cal rallies to beat Cougars 34-28 A's beat Mariners 7-5 in 13 innings Rawls unfazed by chance to be Seahawks’ lead back Juulsen, Skoleski lead Silvertips to 3-1 victory Nats' Scherzer strikes out 17 Mets as he pitches 2nd no-hitter of season Seahawks' Lynch, Mebane to be game-time decisions Monday night No. 12 Clemson holds on to beat No. 6 Notre Dame 24-22
Our new comment system is not supported in IE 7. Please upgrade your browser here.