I think Bortles improvement is threefold.

1) he's no longer dealing with the right wrist injury that had him on the injury report for most of this season
2) he has the full starting 5 offensive line back. Omameh and Parnell are a lot more integral than I wouldve thought, and the unit as a whole has played well above expectations.
3) he has playmakers around him. Fournette alone gives him a better running game. and even with the injuries at receiver the new depth has allowed him a plethora of viable options.

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