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That’s what training camp is for.
You don’t have to sabotage the competition to do this. Have them compete. If Rattler is the guy, a Derek Carr being on the roster shouldn’t prevent him from showing that.
I mean, there is no substitute for live action and you really can't get that in camp or pre-season games playing with soon to be insurance salesmen. If Rattler is ever going to be starting QB in the NFL he needs to get game times with the 1st string to see if he can correct his issues and they are issues that are not going to get better sitting on the bench. And you aren't going to be able to decide if you need to draft a QB in 2026 based on performance in camp or pre-season. And, as you know, there is very little room for a backup to develop during the season.
Regardless, wanting to see what Rattler has is a legit reason to move on from Carr this year.