"We won't be able to create, short term, job opportunities for everybody. Synergies is part of the story," Ermotti said at an event organised by the Asset Management Association Switzerland in Bern.
"We need to take a serious look at the cost base of the standalone and combined organisations and create a sustainable outcome," he added. "It will be painful."
Switzerland's no. 1 bank, which agreed in March to take over its smaller Swiss rival as part of a rescue orchestrated by Swiss authorities, has said it aimed to close the deal quickly.
"Hopefully in the next few days it's going to be done," Ermotti said on Friday. "We are finalising the last the last few miles ... we have more than 170 approvals from regulators."
Ermotti, who led UBS from 2011 to 2020, returned as CEO in April to oversee the biggest banking deal since the global financial crisis.
While he stressed it was a takeover and not a merger, Ermotti said Credit Suisse had many good people and talents, suggesting its executives may play a greater role in the combined group than the initial leadership team unveiled last month might indicate. That management reshuffle only saw Credit Suisse CEO Ulrich Koerner joining the top leadership.
"We will have a more even distribution of jobs ..than the one I did myself," he said. "When the dust settles down ...the best thing for our clients and shareholders and our people is to have the best people in the jobs."
He also insisted that the new combined entity, which will have a balance sheet of $1.6 trillion - roughly double the size of Switzerland's annual economic output - was not too large for Switzerland.
Switzerland's Social Democratic Party has drawn up proposals to shrink UBS assets after its takeover of Credit Suisse to reduce the risk of another expensive state-backed rescue.
"I don't think we are too big for Switzerland," Ermotti said, adding that in banking "size matters."
(Reporting by John RevillEditing by Tomasz Janowski)
By John Revill
