I don't think the position is the issue but the freedom he was allowed behind Di Natale. Sanchez doesn't strike me as a player who thrives in congested space. When he played well at Udinese it was usually because he could dribble against opponents at high speed. The striker absorbed the marking and gave him the opportunity to blast through with a supporting run.
The best solution to get the most out of him would probably be to have him play off Giroud in a two-striker system. But that's not going to happen because it poses a whole new set of problems.
