A number of clubs are thought to be chasing the Brazilian's signature with Chelsea and big-spending Paris Saint Germain believed to be lining up moves for the playmaker.
Lazio are keen to keep hold of Hernanes and Petkovic expects the player to link up with them for pre-season training later this month after his holiday.
"He's on vacation right now and it's only right that we leave him to rest," said Petkovic.
"Has the club reassured me? We all know the transfer market is unpredictable. By my calculations Hernanes will start training with us on July 26.
"I think he will be an added value for Lazio."