Petrachi claims the Cassino-born player could cost in excess of £16million should he impress for Torino in the top flight this season.
"I can guarantee that there are observers of many foreign clubs who follow our games, including United's scouts," Petrachi told tuttomercatoweb.com.
"Most of the time they come just to see Ogbonna because he is a player who has great potential.
"Last year we refused £10million for him, but it is clear that he is worth more than £16million.
"When an offer arrives, the final word will be up to the player. Angelo certainly has the desire to do better, but I believe he should continue to grow with Torino because his primary goal is to improve himself with our team."