We cannot tell what this means, and must rewrite it either:
His disease can only be alleviated by a surgical operation.
Only a surgical operation can alleviate his disease (it cannot be alleviated in any other way).
A surgical operation can only alleviate his disease (it cannot cure it).
The Complete Plain Words, 2nd edn. 1973
I tapped you on your arm with my stick yesterday.
Stress can be used to resolve some potential ambiguity.