Aaron Glenn knows how this business works.

He knows — from his 15 years as a player, from his decade-plus in coaching — how fleeting any window of opportunity in the NFL can be for someone who was a draft pick and especially for someone who wasn’t selected.

Take flight with the Jets

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So Glenn understands what Brady Cook is staring down Sunday, when the undrafted rookie from Missouri will quarterback the Jets in his first career start.

Sometimes, there’s only one shot. Sometimes, it’s the perfect storm of injuries creating an avenue to the field that wouldn’t otherwise exist, which happened for Cook when Tyrod Taylor (groin) and Justin Fields (knee) didn’t

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