The issue is that the #4 LB does indeed need to be able to start, as Collins did last year.
Yes, Beauharnais could be one of our two ST/LB's if he is good enough to play reps at linebacker and a top special teamer, as Fletcher was. Note that the #5 LB played lots of reps last year (Fletcher).
We also need a 4th LB, the first linebacker off the bench.
After all, we lost Fletcher AND Spikes.
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LINEBACKERS (6)
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starters (3) - Mayo, Collins, Hightower
backup (1) - ?????
ST and #2 backup (1) - ????? (could be Beauharnais)
STer and emergency backup (1) - White
THIS IS A MAJOR WEAKNESS
We are counting on
1) Collins as a starter
2) someone new to be the #4 LB (this is a lot to ask of a late round rookie)
2) Beauharnais as a backup and STer
CONCLUSION
We will add a couple of JAG linebackers from free agency, and could consider using a pick in the first 3 rounds on a linebacker or DE/OLB.