I just can't imagine why the bears would let roach go if he is all that great!!
The Bears were always caught up in the aura of Urlacher and Briggs. To be honest, Roach coucd cover much better than Urlacher at the end but you know how it is; how do you keep Urlacher off the field. In my opinion, they made a mistake of letting him go.
The Raiders have said Roach is the MLB and the thing they have been clear about is he will be the guy covering too, so he's in there for the duration and on a bad team that will have its defense on the field a long time, that's fantasy friendly.
If he struggles, they won't pull him because they can't. This team is nothing but liabilties at depth in the LB department.
I wouldn't get too concerned about the week 1 boxscore for him. The Cowboys were running almost exclusively wide off tackle (I assume to give their edge guys some work or see how that was going as they moved their TEs to block). When they weren't doing that, they were googly-eyeing Dez down the field and nobody is crazy enough to send Roach out to cover Dez.
I don't think you will see Roach as a dynamic playmaker like Daryl Washington that does a lot of everything or Sean Lee that seems to be picking people off and forcing fumbles but I think you will see Roach as a great compiler who, at the end of the day, sits on 9-12 tackles, etc. So it will probably depend on your scoring but, at where you can likely draft him, he might be the cheapest 3 down LB you can get that will tally 85+ tackles.