Consider this for a minute. If George McCaskey had to tell Nagy to play Fields, it likely wasn't out of the blue. Remember, these are the "collaboration team" here from last year's press conference.
So what likely happened is that George was hinting to Nagy all this time to play fields, and nagy being stubborn, refused. George deferred to his coach, but when it became apparent that Dalton wasn't it, George put his foot down, since Nagy wouldn't take the hint, and salty nagy hung out Fields to dry, in an effort to convince Geroge to let Nagy start Dalton over Fields when Dalton got healthy.
The irony is, had Nagy just taken the hint instead of being an insecure coach, he likely could have sold Fields growth, and wouldn't be fired by the end of the year.