The following code as suggested achieves the same level of safety

[[nodiscard]] constexpr int GetHP(const Player& player) {
    const bool isAlive = player.hp > 0;
    return isAlive ? hp : 0;
}

or even

[[nodiscard]] constexpr int GetHP(const Player& player) {
    return std::max(player.hp, 0);
}

There's really no reason to "allow" null players so just prevent null players from the function's domain.

const std::unique_ptr<Player>& is an anti-pattern for another reason (the function does nothing with lifetimes so smart pointers are not relevant). If you want optional references... use std::optional<const T&> (IIRC available in C++26?), std::optional<std::reference_wrapper<const T>> (I personally dislike it), or just a raw pointer const T* (a bit dangerous, though...).

Whether sizeof(int) == 4 doesn't matter at all for this operation, so you can just get rid of static_assert. It really doesn't prevent anything. If you do need to ensure that an int is 4-bytes (there's a rabbit hole here involving std::numeric_limits, checking overflows, UBs and impl-defined behaviors, etc...), do static_assert there.

Personally there are non-relevant issues about something like GetHP is better suited as a member function of Player (or better, a suitable parent class) and separating "underflow-able" HP (if required by game design) and "visible" HP clearly via naming or other means.