Mack, who was entering the fifth-year option season of his contract, held out from training camp and the preseason in search of a new deal. Gruden said the negotiations between the Raiders and Mack were "tough, long, and drawn out" and, at the end, something had to give.
"We made an offer, I don't believe we were anywhere close to where the Bears were, and the Bears made us an offer that we thought was really unique," Gruden said.
The Bears landed Mack, a 2020 second-round pick, and a 2020 fifth-round pick in exchange for two first-round picks, a 2020 third-rounder, and a 2019 sixth-round selection.
Simply waiting to see if the team's standout defensive talent would report in time for the regular season was not an option for Gruden.
"We have waited. We waited and waited and the Rams game (season opener) was looming. Our feeling was he was not going to report anytime soon. ... You can wait it out, you can franchise him, you can force him to play, but we made a decision and we're going to stand by it."
Gruden said he "had nothing to do" with the standoff and was not involved in the day-to-day negotiations.
"It wasn't my goal to trade Khalil when we got here. One of the reasons I'm here is because of him," Gruden said. "Unfortunately, we had a standoff with a contract and we could not come to terms. The Bears made us an offer of two first-round draft choices and here we are today."
After getting Mack, the Bears handed the 2016 Defensive Player of the Year a record six-year extension reportedly worth $141 million with $90 million guaranteed.
"There was a standoff and he got a great contract from the Bears. A great contract," Gruden said. "Ninety-million guaranteed is an astronomical number. It's phenomenal for the players, great for him, obviously, but that was something we could not do.
"It's tough to trade him, but there's an economic part of this that's also tough."