After successful hamstring surgery, De Bruyne is expected to be sidelined until at least March.
Asked about the situation, Manna told Sky Italia: "It always takes time for new players to settle in. We knew them well, and we hope they can all earn a place.
"They'll have to convince the manager by showing they're ready. De Bruyne injured? There aren't many players of that quality. We're now looking around, as is only right, and we'll be ready at the right time. A coach's job is always crucial.
"We're coming off a tough defeat in the Champions League, but it takes time: players aren't machines. We hope we've done the best job possible, and the results will help the new players integrate."
On their summer signings, Manna also said: "We hope we've done the best job possible. I think the results also help integrate new players. If things go well, it's easier to fit into a new environment."