Since you started with the obvious 6 you are basically calling our best backup a starter. Including the one who doesn't start from your 6 plus Karras and Herron and thats about as good as one should expect from their depth. That's 8 players who all have played well for you before.
1) The obvious 6 includes Wynn, Onwenu, Karras, Andrews, Mason, Brown
2) I believe that this set of 6 are among the best in the NFL, as it was last year. If this group were all we had, when there would any reason for an OT to leave the game, Onwenu would need to move to OT and Karras would move to LG. This is not optimum.
3) Now let us look at depth, the additional 3 roster spots. Do we have great depth. I am fine if I am wrong. Others can look at the other top 10 OL's and see if their backups are better than ours. The rules now allow 8 active OL's. We are likely to have another inactive, but on the 53.
So, who are the 2 that we are relying on to be active, plus another expected to be inactive.
I celebrate the signing of Ferentz as a Game Day backup. Why? because he, a mediocre player was so needed.
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OUR DEPTH (in addition to Karras) IS THREE OF
Ferentz, Redmond, Herron, Sherman, Toran, Cunnigham and Martin
It is what it is. Perhaps, the issue is semantics. We have one of the top 3 OL's in the NFL, yet we speak of Onwenu switching position during a game or counting on Herron, Cajuste or Cunningham as our swing OT. Personally, don't think that this is great depth.
I understand why Belichick didn't draft an OT until late. He hopes to extend Brown aand/or to have on of the others step up.