According to the latest Wrestling Observer Radio, the reason Sami Zayn hasn't been wrestling lately is because he's been having shoulder issues (something which has been causing him problems for years now). As a result, he's been paired with Shinsuke Nakamura.
There's no word on when Zayn will be able to return to regular in-ring competition but he is doing okay and able to wrestle. He has worked a handful of live events since being paired with the King of Strong Style and even squared off with Buddy Murphy in Australia last month. However, he hasn't competed on TV since August.
Clearly, WWE is anxious to ensure he doesn't do any further damage to his shoulder and are happy with the work he's been doing with Nakamura so chances are this partnership will continue for now.
We'll keep you guys updated.