Sanji is not weak.
He may not be as strong as Luffy or Zoro but he is definitely not weak.
It just happened that he fought with very strong people in this arc.
Maybe he just lost his eye during his training with Mihawk. Maybe there is really nothing it there.
Just like how everyone was saying things about Sanji's left eye only to find out there is nothing weird or different in there except his eyebrow.
