He didn't play much nickel for the Titans. He played in their big Dime package hence why he would often line up on the TE. Your nickel back usually matches up against the slot WR and one of the 2 LBs in your nickel package matches up on he TE.
Not saying he can't play in nickel or play as a high safety but that would be something he hasn't traditionally done before. Looking at his measurable and reading his scouting report, he definitely would appear to have the speed and explosion to be able to match up against WRs or play as a deep safety.
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Don't think you want a SAM backer that is only 209 pounds especially when Quan and Morrow are also undersized. Eberflus likes versatility so he now has 3 safeties that can also drop down and cover in man if need be. I suspect DC and DHC battle it out for the other starting S spot in base D and that all 3 play some nickel or dime. Jackson probably gets the nod against slot WRs and DC against bigger slot WRs or TEs. NB is one of the more important positions in a 4-3 so I can see them wanting to train all 3 safeties in that spot as well as Graham and Vildor assuming someone else gets the nod at CB2.