Earl Thomas.
I hate to say it, but CB is a system position for Carroll. He can plug anyone in there who fits his system and they will do well. For example, look at all the no-name CBs who played for him at USC and look how many CBs have suddenly found success playing for us.
Did anyone notice a significant drop-off when Browner was out? No, because all the guys on the team fit Carroll's system and they can perform at a high level in it.
Sherman is our best CB, and there would be somewhat of a drop-off if he were gone just because he fits Carroll's system so PERFECTLY, but it would not be devastating.
Thomas, however, plays a very critical position in Carroll's defense and would be harder to replace. It's why Carroll went after a safety in the 1st round of his first draft (remember there were rumors that he wanted Eric Berry with his first pick, but he was gone before we took Okung) and why he has never drafted a CB before round 4.