The LB that Arrington's production will probably take most away from is Holdman. Arrington played extensively vs. the 49'ers and Marshall still had a sack and several tackles. The MLB in that defense is money.
However, it seems that the team may be using more of a hybrid 4-3/3-4 set with Arrington coming in to play one of the end positions on passing downs. What they usually do then is remove the DT opposite Griffin - usually Joe Salavea - and slide either DE Phillip Daniels or DE Renaldo Wynn over to play DT to make room for Arrington.
We'll see more of this 3-4 set as long as they have multiple DT's injured. Right now, all five DT's on the roster have reported injuries, with Griffin's looking the least severe; only he and Salavea suited up vs San Francisco.
So, when they go to the 3-4, is Marshall still the MLB? It just seems wierd that Arrington would get 9 tackles from a rushing DE position and no sacks.
Yes, Marshall is still the MLB. As for Arrington's tackle numbers, I agree that that's atypical, but it happened in his first game back when I predicted that he'd be all fired up (he was). Also, San Fran discovered very early that they weren't going to be successful passing, so they ran a lot more and Arrington got a lot of penetration and tackles that way. Finally, he played virtually every down, and with second stringers in the second half.
The point is don't downgrade Marshall just yet. Arrington's playing time is a week to week evaluation for the time being.