I've seen Erden play. Far from awful. Raw, yes. Awful, No.
I see a prospect with a super soft touch around the basket, 15 ft range from the outside with a good look, and nimble feet for a big guy. He can rebound, and provides a shot blocking presence at times. He'll run the floor. Now develop him. He's quick off that screen and roll, even though he still needs work on his screens, and some other fundamentals that involve more technique than skill. Shape up the technique with some coaching, and 2-3 years down the road he'll get even more time out there on the floor.
My guess is he plays the Stojko Vrankovic role out there this year (a lot of DNP's, minutes here and there during blow outs, etc.), while he continues to develop his game. I think Ainge finds other veterans that take his role for now.