@dplank
Short answer: up to 13.144M. Long answer below for anyone who wants to know the mechanics.
Most obvious place to defer some 2022 cap space for Mack is a 5.5M roster bonus he has which becomes due March 11. Without any extra void years, it can be restructed over 4 years @ 1.375M, opening up 4.125M space in 2022 and increasing the cap hit for 2023-2025 @ 1.375 per year. There is no real risk to this IMO, and I think its a no brainer move, personally.
If they can add an additional void year, they can spread it over 5 years, opening up 4.4M in 2022 and increase cap hits of 1.1M in 2023 & 2024 and 2.2M in 2025*
He also has a 12.05M base salary, that is unguaranteed. Teams don't typically like to restructure unguaranteed salaries, but you can. Once week 1 starts however most veteran contracts become fully guaranteed. You saw a lot of teams restructure contracts like this during the year this year. The Bears notably did it with about half of Quinn's base salary this year to give them extra breathing room, about half of which they did end up needing for in season moves.
If the Bears did restructure the full base salary of 12.05M they could defer 8.197M (no extra void years) or 8.744 (add max void years). To be clear, that can be done in addition to the roster restructure mentioned above. Keep in mind if you do these moves in season you lose 1/18 of your ability each week as weekly salary is paid and can then no longer be converted.
*NOTE. Any void years after the final contract year automatically vest (unless extended,) in the year after the last contract year. Right now Mack's last contract year is 2023 and he already has a void year in 2024, but they can add void years only with the player's approval, as its considered an extension. A normal restructure can normally be done without a players approval if they have "automatic conversion" language, which I understand most veteran contracts do. Automatic conversion language is not allowed in rookie scale contracts FWIW. So you rarely see rookie scale restructures, FYI.