The Lions offense heavily utilizes TE's, often employing 2 TE sets. This is a point which can't be ignored. However, in Pettigrew and Scheffler you have two guys who are mid-career that the Lions can choose to resign, Franchise Tag, or sign another free agent.
At C and G, you have a position in a real state of flux. Raiola is very clearly and the very tail end of his career. As the QB of the O-Line, Raiola makes an impact on every single play with his protection calls and veteran awareness, something not easy to replace. At the moment, there is also a very real depth issue with Peterman being gone. A lot of people are pinning their hopes on the oft-injured, 7th round compensatory pick, and 4 game NFL starter Bill Nagy to fill the void. Is that really a good strategy? There is also a lot of discussion regarding shifting Reiff inside, again, not a guarantee given they may need him to start at either RT or LT.