I've been wanting the Lions to trade Stafford for years, but I'm resigned to the fact that it isn't going to happen.
With that assumed to be the case, they don't need to draft Tua, but they better take the best offer they get to trade down.
I think the absolute best news for Lions fans this off-season would be:
1) Tua's health checks out, and he looks to be ready to go at least at some point this season (and appears to be on the road to a complete recovery).
2) Phillip Rivers signs a contract with someone other than the Chargers or retires, and the Chargers don't sign a long-term free agent (a short deal wouldn't scare me).
3) The Panthers trade Cam Newton.
With those two things, that #3 pick becomes awfully valuable, hoping that the Dolphins, Panthers, and Chargers all could potentially get into a bidding war to trade up with the Lions to draft Tua.
But since we are talking about the Lions, I expect all three of those things to happen, the Lions draft Okudah, and the Giants make a blockbuster trade at #4.