The wording for the piggyback certificate requires your underlying licence to be valid, which is partly what the medical achieves.
So if your "foreign" medical is current, you're OK. You don't need an FAA medical as well.
Here's the wording for
14 CFR Part 61, Section 61.75 which says in part (note the "or"):
Originally Posted by 61.75
(4) Holds a medical certificate issued under part 67 of this chapter or a medical license issued by the country that issued the person's foreign pilot license;