If it is a slipped tendon it won't heal on its own. Tendons are stretchy and hold things in place. In a slipped tendon, the tendon stops being stretched and shortens. It takes time for it to gain that stretch again. In a fast growing animal like a duck, it can become nearly impossible to stretch...
https://www.backyardchickens.com/threads/please-help-me-im-desperate-duckling-with-a-slipped-tendon.964187/
Here is a good one. It has links to good information. Some talk about chickens, but bird's legs don't differ much.
Make sure it is easy for the duckling to access water and food. I would give the duckling scrambled eggs.
Also check the leg to see if you feel any breaks.
I haven't had this problem with a duckling before. @Miss Lydia or @Isaac 0 may have other things to suggest specific to ducklings.