The difference? Who knows exactly? Could be a dramatic difference, but also one that might be difficult to quantify.
The signing? It is a wise, precautionary move—providing cover, should Yoshida or Stephens get injured. He might even become a first team starter, if he gets fit and adapts quickly.
Ultimately, it must be better to have him in the squad than not. Regardless of how things turn out, he is obviously a "relevant" signing.