He ran a fairly advanced passing attack at Ohio State at an elite level. Ain't no way this dude went from that to Tyler Huntley overnight. The problem is he's been harassed fairly frequently.
This was an article from Oct 21
How Justin Fields is hurt by constant pressure from Bears offensive line - NBC Sports Chicago
According to PFF, Lucas Patrick has allowed 11 pressures in 57 true pass set opportunities (passes excluding play action, screens, short drop backs, plays with fewer than four rushers, and plays where the time-to-throw is under two seconds). That’s ninth most among all guards, but his 19.3% pressure rate is second highest at the position (minimum 50 opps). Sam Mustipher has allowed seven pressures in 69 TPS opportunities. That’s the fifth highest total, and his 10.1% rate is second highest among centers. Braxton Jones has been the worst offender with 17 pressures allowed in 69 TPS opportunities. That total is fourth highest among all tackles, and again, his 24.6% pressure rate is second. Teven Jenkins hasn’t been perfect, surrendering five pressures in 52 TPS opportunities, but he’s well down the list at guard. Larry Borom has been the best, giving up only four pressures in 69 TPS opportunities.
Unsurprisingly, Fields has been a very different quarterback when he’s pressured compared to when he’s kept clean.
Pressured: 33.3% completion rate, 6.9 YPA, 2 TD, 3 INT, 22 sacks, 43.8 passer rating
Clean: 65.8% completion rate, 7.9 YPA, 2 TD, 2 INT, 1 sack, 87.6 passer rating
And then also add in that his best WR this year was Mooney, who went on the IR at some point mid season. Like when they played the Bills they had Claypool, Dante Pettis and Eq. St Brown which is simply not an NFL caliber set of WRs