You can use
Meine Deutschkenntnisse sind schon etwas eingerostet.
My German is a little rusty.
In English - if I'm not mistaken - it's possible to attribute "rusty" to people:
I'm a little rusty [at something].
Although I hear it occasionally,
I'm not quite sure the same applies to German. To me, it feels more natural to label the respective skill/language as rusty. On the other hand, this dialogue seems fine:
Kannst du noch Basketball spielen?
-- Puh, ich bin schon etwas eingerostet.
Although in this case, the "rustyness" does not explicitly refer to basketball, rather to his/her overall fitness, as Em1 pointed out.