Mack, who is in search of a new contract, will not report to training camp when it opens Thursday, per Schefter.
Schefter added that while he believes Gruden has "tried" to reach out to Mack, he "hasn't succeeded."
Per ESPN.com, Gruden spoke highly of Mack in June, saying, "One of the big reasons I came here was to coach that man."
The 27-year-old Mack is a three-time Pro Bowler and two-time All-Pro First Team selection during his four-year NFL career.
He was named the 2016 NFL Defensive Player of the Year and is coming off a 2017 season that saw him register 78 tackles and 10.5 sacks on an Oakland defense that otherwise struggled.
In 64 career regular-season games, the 2014 No. 5 overall pick out of the University at Buffalo has 40.5 sacks to his credit.
Mack is set to earn over $13.8 million in 2018, which is the final year of his current deal, according to Spotrac.
A long-term extension isn't necessary from Oakland's perspective since it can use the franchise tag on Mack, but it would go a long way toward ensuring that his holdout doesn't carry over to the regular season.
Since the Raiders tied for 24th in the NFL in sacks last season even with Mack, getting him back in the fold for the start of the 2018 campaign is paramount if they are going to return to the playoffs after missing out in 2017.