It's really quite simple. Poz and Telvin were the dead last nickel duo LB in yards allowed last year. They are both quite bad against the pass, even though they are both quite bad in the exact opposite ways. It's just the truth, sorry. 

 

Unfortunately, Poz is the only person on the team right now that can adequately line up the defense...so he stays on the field until someone else can possibly do it. 

 

That means if Jack is better in coverage than Telvin (he will be very quickly if he isn't already), then he should be the other nickel linebacker alongside Poz.

 

Once the team feels that Jack can get the defense adjusted properly then they probably will have Jack sub for Poz instead. 

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