I don't think Rivera is a question mark. Even though he was stellar last year, he certainly was better than average. Not what we paid for, but still not bad. He should only get better this season.
I also don't think the Center position was terrible last year. They did fine in pass protection, i just think that Al Johnson does nothing in the run game.
Kosier seems to be a lunch pale, hard working, technically sound OL. Maybe not the most talented, but seems to work and play hard. Probably the polar opposite of what we had last year. This guy does have upside.