IMHO, if it can still fly, it'll be ok against attacks.

Would not recommend intervention.

If you cannot see swelling, bleeding or any apparent injury, maybe you could watch the situation for a couple of days, while continuing providing food, and see how it evolves. If it worsens, then contact the rehab center. Probably it is a minor injury and will heal on its own.

In any case, the duck is lucky to have you around.