I think this is the worst case.
We went through that scenario with Cam coming off the injury in 2019. We needed to see if he had regained his health/throwing velocity before committing to another contract. Then a different injury hit and that became impossible, at least on the timeframe the team had.
No injury this time around, but still an evaluation. Darnold going down early would make that next to impossible.
As somebody said, Darnold playing well enough to for us to flirt with .500 is not a bad scenario, assuming he shows enough that coaching staff concludes he is on an upward trend and has not yet approached his ceiling. It was pretty obvious with TB that the staff saw what they believed was his ceiling.
Darnold is not really any different than a newly-drafted QB, except that he has a few years of seeing the speed of the NFL game first hand. A lot of people underestimate how different that is. That should allow a quicker evaluation.......assuming he does not get hurt early. Given he is coming to the end of his rookie deal, the evaluation has to be made quickly. But, I am not sure that does not go on with any young QB these days. More is expected sooner, and that is why the hit rate for even serviceable NFL QBs appears to have gone down.