The German has scored eight times in the Bundesliga since moving from the Olympic Stadium, finding the back of the net in six of his last seven games for Hamburg.
He is one of three players tied for second place in the league's scoring charts, but Preetz is confident Lasogga will return to Hertha in the close season.
"Hamburg has no option to buy," he told Bild, "Pierre will return to us in the summer of 2014."
However, Preetz's opposite number at Hamburg, Oliver Kreuzer, has not given up hope of signing the 21-year-old permanently.
He also feels the decision will ultimately come down to where Lasogga feels his future lies.
"Let's speak in due time to Hertha's coach and the players on this issue," he said.
"I think that the decision, while Pierre's in the end, falls late in the season."
