Must Justice Kagan Recuse Herself from NLRB Case?

A lawyer friend points out:

In the aftermath of Friday's DC Circuit decision on Recess Appointments, the expectation is that the Supreme Court will hear the case.

But it appears that one Justice may be barred from hearing the case because she worked on the very issue while Solicitor General of the United States, i.e., Elena Kagan. Kagan was all over the issue at DOJ. What she did was perfectly legal and proper. But, she may be barred from hearing argument/participating because of her direct role.