The rating agency sees Invepar's capital structure as "unsustainable."
A consortium formed by an engineering company and an investment fund offered around US$30mn for the 35-year concession.
The US$386mn project, awarded as a turnkey contract, entails resurfacing a 277km road stretch connecting Boquerón and Alto Paraguay departments.
After the concessionaire said it would sell an 85% stake and two investment funds signaled interest, Moody's put the global and the local ratings under review.
The country's transport ministry highlighted that it has managed to resolve bottlenecks in seven projects in the 4G highways program so far.