Gruden told reporters Tuesday that the move had to happen despite Mack's talent.
"We're not able to get Antonio Brown, Trent Brown and Lamarcus Joyner without making that trade," Gruden said. "I wanted to coach Khalil Mack. He knows that."
The Raiders acquired star wideout Antonio Brown from the Pittsburgh Steelers for a third- and fifth-round selection in the 2019 NFL Draft this off-season. The franchise then restructured Brown's contract, giving the wideout a pay raise.
Oakland added offensive tackle Trent Brown and defensive back Lamarcus Joyner in free agency, moves that likely couldn't have happened with Mack's new salary. The Bears gave the 28-year-old linebacker a six-year, $141 million contract extension after the trade was completed.
The Raiders have the fourth-overall pick in the upcoming draft after finishing with a 4-12 record last season in Gruden's return at the helm. Oakland has three first-round picks in April's draft.
Meanwhile, Mack led the Bears with 12.5 sacks while earning Pro Bowl and All-Pro honors in 2018. Chicago finished with a 12-4 record before losing to the Philadelphia Eagles in the NFC Wild Card Round.