This was Del Rio's second try as a head coach after spending nine seasons with the Jaguars from 2003-2011.
Raiders owner Mark Davis informed Del Rio after Sunday's loss to the Chargers that he has been fired as head coach.
In three years in Oakland, Del Rio was 25-23.
The 54-year-old had signed a four-year contract extension back in February, but with the Raiders taking a step back this season missing the playoffs finishing with 6-10 record led to his firing. Stories about Gruden, who hasn't coached in a decade, returning to the sideline have become an annual tradition, but perhaps this time there's actually some traction to them.
Dirk Koetter to return as Bucs coach in 2018
In his two seasons as the team's head coach, Koetter sports a 13-18 record after a disappointing 2017 season. Following the Bucs' loss to the Carolina Panthers on Sunday, Tampa might want to give that pair back.
The Raiders followed that up with one of the most disappointing seasons in the NFL.
Earlier on Sunday, it was reported that Gruden was vetting coordinators and former players to take spots on his coaching staff. The defense didn't get better and they had to fire defensive coordinator Ken Norton during the season.
"I spoke to Mark Davis after the game and Mark let me know that he's not going to be bringing me back", Del Rio said at the podium. The Raiders had a breakthrough season in 2016 with 12 wins but it ended in disappointment when a broken leg for quarterback Derek Carr in Week 16 cost the Raiders a chance at a division title and led to a first-round playoff loss.