Another way of looking at things is which position would be most weakened by the loss of the starter?
If you combine your thinking in terms of available PT by with where the depth is most deficient there are some crossovers between both approaches.
In the most weakened line of thinking on D both NT (nobody on roster after Mebane) and 3 tech (are we all in on Bennett as the 3 tech with a torn Labrum?) are need areas as is WOLB, and NCB now we have Winfield. It can also be argued the team needs a better backup FS than Maragos.
On O besides the obvious ? Of what happens at QB if RW is out for any time. Is there enough depth at RB assuming injury to either of Lynch or Turbin? Is there a go to TE if Miller is out for an extended time? Is there any realistic depth at OT on the team? Does the team have a realistic backup big WR if Rice went down?
With this thinking the positional areas where depth might need to be added in no particular order are: DT (NT & 3 tech), WOLB, FS, & NCB on D. While on O: OT, QB,TE, big WR might be depth areas. The team is left to take a flyer or trade away one pick to hopefully recover the 3rd for next years draft lost in the Harvin deal.
Which pick happens at 56 depends on the best value available then still on the board.
Gotta say I think you are on to something. Today before I read this I had already started to lean toward DT as being more important than WLB. Scruggs and Howard could step up, but adding a big body to stuff the run and creat pressure is still an area of weakness and one that has haunted us for the last two years. Even when Jones was healthy that was not completely fixed. I think JS & PC are going to have a tought time deciding between the talent available at DT and L, not to mention the possible WR's, TE's, and OL that could be ther at #56.