I believe that the QB has to fit the system. I know "system" QB is a pejorative, but the best teams have a match between the two. So whatever version of the Bears offense they are going to have with a new OC should be put in place before they make decisions about who to draft when.
If they honestly assess Fields and think he can do the job if put into the correct system, I can accept that, so long as they get a good OC/system fit for the things he does well and try to minimize the things he doesn't do well.
However, he has not shown the consistency needed to be a starting QB, no matter the system. I also don't think Poles is going to stake his job on it, but you never can tell.