Sami Zayn is working as a manager to Cesaro and Shinsuke Nakamura because of a creative decision, not because of an injury.
There’s been speculation on WWE putting Sami in the managerial role on SmackDown because of the shoulder issues he dealt with last year and the year before, but that is not the case, according to Dave Meltzer on Wrestling Observer Radio.
Sami is currently not injured but he’s not wrestling regularly because this is the role WWE creative has for him right now. Sami was reportedly medically cleared to compete from his shoulder issues a while back.
Sami has worked very few matches since late summer 2019 – a few live event multi-man matches, and a SmackDown multi-man dark match at the end of December. It’s possible that WWE is keeping him out of the ring as a precautionary measure because of his shoulder issues, but he’s been cleared to wrestle for a while now.