Not quarterback. Not offensive line. The secondary.
Think about it, other than cornerback Sam Madison, what do the Dolphins really have in terms of proven commodities?
The answer is not much.
The closest thing the Dolphins have to a sure thing is strong safety Tebucky Jones, the former first-round pick of New England. But you also must remember that Jones joined the Dolphins after being released by New Orleans, which isn't exactly in a position to jettison quality defensive backs.
Tillman is a former second-round pick who hasn't lived up to expectations so far in his career, and any time he might miss at the start of training camp would hurt. It's too early to determine exactly when Tillman will be ready to work, but there's no guarantee he'll be at it when camp opens in late July.
Tillman is expected to join Jones at safety in the starting lineup, replacing Sammy Knight and Arturo Freeman, who ended last season as the starters at those positions.
Until proven otherwise, the switch can't be considered an upgrade in talent.
There isn't much experienced backup help, either, because Yeremiah Bell hasn't been able to hold off injuries.
The situation isn't that much more promising at cornerback, especially after the season-ending knee injury sustained by Will Poole.
At it stands out, either Mario Edwards, Reggie Howard or rookie Travis Daniels will join Madison in the starting lineup. That spot will continue to be considered a major question mark, unless Ty Law reduces his contract demands and the Dolphins can sign him.
Nick Saban is expected to make a big difference on defense because the Dolphins won't be so vanilla anymore, but Saban still needs players to make his system work.
There's reason to wonder whether the Dolphins have enough in the secondary at this point, and that's why any little injury for a defensive back is that much more disappointing.