BUT…
If the federal court needs to create a federal common law rule to address a matter in a federal question case, it may find it convenient to borrow a standard from state law.
E.g. when federal statutes lack their own limitations periods, federal courts commonly borrow the limitations period from an analogous statute of limitations of the forum state.

AND…
It is still possible that the federal law purporting to regulate the matter is invalid.
for example…
A Federal Rule of Civil Procedure might be invalid because it abridges, enlarges or modifies a federal substantive right, in violation of the Rules Enabling Act, 28 U.S.C. § 2072(b).
An example would be Fed. R. Civ. P. that purported to establish a duty of care. 
If the Fed. R. Civ. P. is invalid for this reason, then the federal court would have to come up with a federal common law rule to address the matter that respected the relevant federal substantive right.