While Upshaw is listed as a linebacker in Mayock's mock draft, chances are he would play defensive end for the Lions. He projects as a 3-4 linebacker and a 4-3 defensive end. Obviously the Lions could use him in different ways, but chances are he would primarily have his hand on the ground.
NFLDraftScout.com actually thinks Upshaw is falling in the draft a bit because of his tweener status:
Some teams will value him as a 4-3 end while others might see him as a stand-up linebacker in a 3-4. Then there might be others who see neither because he has too many "tweener traits" to be taken in the first round. In Upshaw's defense, he was an impact defender for the National Champions of college football last year and does an excellent job with his hands and angles to make a play on the ball. While he is still projected to hear his name in the first round, most don't believe he is an "elite" pass rusher in this draft class.
I wouldn't be too upset with this pick if Whitney Mercilus is gone. In Mayock's mock, Mercilus is projected to go 18th overall to the San Diego Chargers. Nick Perry, who many believe is the next-best defensive end after Mercilus, is projected to go No. 27 to the New England Patriots.