Del Rio was the Broncos' defensive coordinator. He previously was the head coach of the Jacksonville Jaguars from 2003-11, building a 68-71 record. Del Rio was the Jaguars' defensive coordinator when Fox was the head coach in Jacksonville.
Del Rio is to continue in the role as Denver's defensive coordinator.
Fox, 58, was playing golf in North Carolina during the Broncos' bye week when he said he was feeling dizzy. He was taken to a hospital and was diagnosed with a heart ailment. He had an aortic valve replacement procedure Monday, the team said, and will need several weeks to recover.
"Our family greatly appreciates the overwhelming support and well-wishes we've received for John," Fox's wife Robin said in a statement from the Broncos. "Along with his medical team, we will take great care of him so he can fully recover and get back to coaching the Broncos as soon as possible."
Denver is 7-1 this season and 1 1/2 games behind the Kansas City Chiefs in the AFC West. The Broncos are to be play San Diego (4-4) Sunday.